Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 16, 6:18 am, Steven D'Aprano  wrote:

> On Sun, 16 Aug 2009 01:41:41 -0700, Douglas Alan wrote:

> > I would definitely not like a language that obscures assignment by
> > moving it over to the right side of lines.

> One could argue that left-assigned-from-right assignment obscures the
> most important part of the assignment, namely *what* you're assigning, in
> favour of what you're assigning *to*.

The most important things are always the side-effects and the name-
bindings.

In a large program, it can be difficult to figure out where a name is
defined, or which version of a name a particular line of code is
seeing. Consequently languages should always go out of their way to
make tracking this as easy as possible.

Side effects are also a huge issue, and a source of many bugs. This is
one of the reasons that that are many functional languages that
prohibit or discourage side-effects. Side effects should be made as
obvious as is feasible.

This is why, for instance, in Scheme, variable assignment as an
exclamation mark in it. E.g.,

   (set! x (+ x 1))

The exclamation mark is to make the fact that a side effect is
happening there stand out and be immediately apparent. And C++
provides the "const" declaration for similar reasons.

> In any case, after half a century of left-from-right assignment, I think
> it's worth the experiment in a teaching language or three to try it the
> other way. The closest to this I know of is the family of languages
> derived from Apple's Hypertalk, where you do assignment with:
>
> put somevalue into name

That's okay with me, but only because the statement begins with "put",
which lets you know at the very beginning of the line that something
very important is happening. You don't have to scan all the way to the
right before you notice.

Still, I would prefer

   let name = somevalue

as the "let" gives me the heads up right away, and then immediately
after the "let" is the name that I might want to be able to scan for
quickly.

|>ouglas
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 16, 8:45 am, MRAB  wrote:

> No, APL is strictly right-to-left.
>
>      -> x
>
> means "goto x".
>
> Writing to the console is:
>
>      [] <- myVar
>
> Reading from the console is:
>
>      myVar <- []

Ah, thanks for the correction. It's been 5,000 years since I used APL!

|>ouglas
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 16, 4:48 am, Erik Max Francis  wrote:
> Douglas Alan wrote:
> > Personally, my favorite is Lisp, which looks like
>
> >    (set! y (+ y 1))
>
> For varying values of "Lisp."  `set!` is Scheme.

Yes, I'm well aware!

There are probably as many different dialects of Lisp as all other
programming languages put together.

|>ouglas
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 16, 4:22 am, Steven D'Aprano  wrote:

> I don't like normal assignment. After nearly four decades of mathematics
> and programming, I'm used to it, but I don't think it is especially good.
> It confuses beginners to programming: they get one set of behaviour
> drilled into them in maths class, and then in programming class we use
> the same notation for something which is almost, but not quite, the same.
> Consider the difference between:
>
> y = 3 + x
> x = z
>
> as a pair of mathematics expressions versus as a pair of assignments.
> What conclusion can you draw about y and z?

Yeah, the syntax most commonly used for assignment today sucks. In the
past, it was common to see languages with syntaxes like

   y <- y + 1

or

   y := y + 1

or

   let y = y + 1

But these languages have mostly fallen out of favor. The popular
statistical programming language R still uses the

   y <- y + 1

syntax, though.

Personally, my favorite is Lisp, which looks like

   (set! y (+ y 1))

or

   (let ((x 3)
 (y 4))
 (foo x y))

I like to be able to read everything from left to right, and Lisp does
that more than any other programming language.

I would definitely not like a language that obscures assignment by
moving it over to the right side of lines.

|>ouglas

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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 16, 1:05 am, Steven D'Aprano  wrote:
> On Sat, 15 Aug 2009 20:00:23 -0700, Douglas Alan wrote:
> > So, as far as I can tell, Python has no real authority to throw stones
> > at C++ on this little tiny particular issue.

> I think you're being a tad over-defensive.

Defensive? Personally, I prefer Python over C++ by about a factor of
100X. I just find it a bit amusing when someone claims that some
programming language has a particular fatal flaw, when their own
apparently favorite language has the very same issue in an only
slightly different form.

> the poster using it has (as far as I know) no official capacity to speak
> for "Python"

I never thought he did. I wasn't speaking literally, as I'm not under
the opinion that any programming language has any literal authority or
any literal ability to throw stones.

> Now that I understand what the semantics of cout << "Hello world" are, I
> don't have any problem with it either. It is a bit weird, "Hello world">> cout
> would probably be better, but it's hardly the strangest design in
> any programming language, and it's probably influenced by input
> redirection using < in various shells.

C++ also allows for reading from stdin like so:

   cin >> myVar;

I think the direction of the arrows probably derives from languages
like APL, which had notation something like so:

 myVar <- 3
 [] <- myVar

"<-" was really a little arrow symbol (APL didn't use ascii), and the
first line above would assign the value 3 to myVar. In the second
line, the "[]" was really a little box symbol and represented the
terminal.  Assigning to the box would cause the output to be printed
on the terminal, so the above would output "3".  If you did this:

 [] -> myVar

It would read a value into myVar from the terminal.

APL predates Unix by quite a few years.

|>ouglas
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 15, 10:19 pm, Steven D'Aprano  wrote:

> On Sat, 15 Aug 2009 13:01:43 -0700, Douglas Alan wrote:
> > P.S. I find it strange, however, that anyone who is not okay with
> > "abusing" operator overloading in this manner, wouldn't also take
> > umbrage at Python's overloading of "+" to work with strings and lists,
> > etc. Numerical addition and sequence concatenation have entirely
> > different semantics.
>
> Not to English speakers, where we frequently use 'add' to mean
> concatenate, append, insert, etc.:

That is certainly true, but the "+" symbol (pronounced "plus" not
"add") isn't exactly synonymous with the English word "add" and is
usually used in, technical circles, to refer to a function that at
least meets the properties of an abelian group operator.

Also, programming languages (other than Perl) should be more precise
than English. English words often have many, many meanings, but when
we are talking about types and operations on types, the operations
should generally have more specific semantics.

In any case, let's say we grant that operators should be allowed to be
as sloppy as English. Then we should have no problem with C++'s use of
"<<" for i/o. Pseudo-code has a long heritage of using "<-" to
indicate assignment, and there are a number of programming language
(e.g., APL) that use assignment to the output terminal to indicate
writing to the terminal.  C++'s usage of "<<" for output is clearly
designed to be reminiscent of this, and therefore intuitive.

And intuitive it is, given the aforementioned background, at least.

So, as far as I can tell, Python has no real authority to throw stones
at C++ on this little tiny particular issue.

|>ouglas
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Re: Python- javascript

[Douglas Alan]

On Aug 15, 8:02 pm, Mike Paul  wrote:

> I'm trying to scrap a dynamic page with lot of javascript in it.
> Inorder to get all the data from the page i need to access the
> javascript. But i've no idea how to do it.

I'm not sure exactly what you are trying to do, but scraping websites
that use a lot of JavaScript are often very problematic. The last time
I did so, I had to write some pretty funky regular expressions to pick
data out of the JavaScript. Fortunately, the data was directly in the
JavaScript, rather than me having to reproduce the Ajax calling
chain.  If you need to do that, then you almost certainly want to use
a package designed for doing such things. One such package is
HtmlUnit. It is a "GUI-less browser" with a built-in JavaScript engine
that is design for such scraping tasks.

Unfortunately, you have to program it in Java, rather than Python.
(You might be able to use Jython instead of Java, but I don't know for
sure.)

|>ouglas


P.S. For scraping tasks, you probably want to use BeautifulSoup rather
than urllib2.


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Re: Python or ActionScript 3.0

[Douglas Alan]

On Aug 15, 5:32 pm, Jaseem  wrote:

> Is python similar to actionscript 3.0

For some very rough sense of "similar" it might be, but not really.

> Which is better to create a rich gui internet application?
> Is it AS 3.0 with flex or python with its GUI libs?

Python doesn't run in your typical web browser, but it is common to
use Python for doing the server-side programming, along with a Python-
based web development framework, such as Django.

You could use Jython to make a JVM applet that would run in a browser,
but JVM applets aren't very popular for a variety of reasons, and I
doubt the performance would be very good.

> Is python in demand?

Yes, it's a popular language.

> Heard that python is similar to lisp.

Kind of. Not any more so that JavaScript is, though, for instance.

> But both python and AS 3.0 is almost identical.

No, Python and ActionScript are not "almost identical".

> Which is more similar to lisp are powerful?

They both have their similarities and differences from Lisp. I think
it would be impossible to say which one is more similar to Lisp. In
general, Python is in my opinion more pleasant to program in than
ActionScript, but Python is not generally used for client-side browser
code.

I think the future of client-side browser programming is actually
JavaScript, not ActionScript, though that future may morph into one
that mostly uses JavaScript as a virtual machine. This is the approach
that Google Web Toolkit takes. It lets you write your client-side code
in Java, and that is then compiled into JavaScript.

|>ouglas



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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 14, 1:55 pm, Steven D'Aprano  wrote:

> Douglas, you and I clearly have a difference of opinion on
> this. Neither of us have provided even the tiniest amount
> of objective, replicable, reliable data on the
> error-proneness of the C++ approach versus that of
> Python. The supposed superiority of the C++ approach is
> entirely subjective and based on personal opinion instead
> of quantitative facts.

Alas, this is true for nearly any engineering methodology or
philosophy, which is why, I suppose, Perl, for instance,
still has its proponents. It's virtually impossible to prove
any thesis, and these things only get decided by endless
debate that rages across decades.

> I prefer languages that permit anything that isn't
> explicitly forbidden, so I'm happy that Python treats
> non-special escape sequences as valid,

I don't really understand what you mean by this. If Python
were to declare that "unrecognized escape sequences" were
forbidden, then they would be "explicitly forbidden". Would
you then be happy?

If not, why are you not upset that Python won't let me do

   [3, 4, 5] + 2

Some other programming languages I've used certainly do.

> and your attempts to convince me that this goes against
> the Zen have entirely failed to convince me. As I've done
> before, I will admit that one consequence of this design
> is that it makes it hard to introduce new escape sequences
> to Python. Given that it's vanishingly rare to want to do
> so,

I'm not so convinced of that in the days of Unicode. If I
see, backslash, and then some Kanji character, what am I
supposed to make of that? For all I know, that Kanji
character might mean newline, and I'm seeing code for a
version of Python that was tweaked to be friendly to the
Japanese. And in the days where smart hand-held devices are
proliferating like crazy, there might be ever-more demand
for easy-to-use i/o that lets you control various aspects of
those devices.

|>ouglas
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 14, 10:25 pm, Dave Angel  wrote:

> Benjamin Kaplan wrote:

> > On Fri, Aug 14, 2009 at 12:42 PM, Douglas Alan wrote:

> >> P.S. Overloading "left shift" to mean "output" does indeed seem a bit
> >> sketchy, but in 15 years of C++ programming, I've never seen it cause
> >> any confusion or bugs.

> > The only reason it hasn't is because people use it in "Hello World". I bet
> > some newbie C++ programmers get confused the first time they see << used to
> > shift.

People typically get confused by a *lot* of things when they learn a
new language. I think the better metric is how people fare with a
language feature once they've grown accustomed to the language, and
how long it takes them to acquire this familiarity.

> Actually, I've seen it cause confusion, because of operator precedence.  
> The logical shift operators have a fairly high level priority, so
> sometimes you need parentheses that aren't obvious.  Fortunately, most
> of those cases make compile errors.

I've been programming in C++ so long that for me, if there's any
confusion, it's the other way around. I see "<<" or ">>" and I think I/
O. I don't immediately think shifting. Fortunately, shifting is a
pretty rare operation to actually use, which is perhaps why C++
reclaimed it for I/O.

On the other hand, you are right that the precedence of "<<" is messed
up for I/O. I've never seen a real-world case where this causes a bug
in C++ code, because the static type-checker always seems to catch the
error. In a dynamically typed language, this would be a much more
serious problem.

|>ouglas

P.S. I find it strange, however, that anyone who is not okay with
"abusing" operator overloading in this manner, wouldn't also take
umbrage at Python's overloading of "+" to work with strings and lists,
etc. Numerical addition and sequence concatenation have entirely
different semantics.
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Re: OT Signature quote [was Re: Unrecognized escape sequences in string literals]

[Douglas Alan]

On Aug 14, 12:17 pm, Grant Edwards  wrote:

> On 2009-08-14, Steven D'Aprano  wrote:

> > On Fri, 14 Aug 2009 07:07:31 -0700, Aahz wrote:

> >> "I saw `cout' being shifted "Hello world" times to the left and stopped
> >> right there."  --Steve Gonedes
>
> > Assuming that's something real, and not invented for humour, I presume
> > that's describing something possible in C++. Am I correct?
>
> Yes.  In C++, the "<<" operator is overloaded.  Judging by the
> context in which I've seen it used, it does something like
> write strings to a stream.

There's a persistent rumor that it is *this* very "abuse" of
overloading that caused Java to avoid operator overloading all
together.

But then then Java went and used "+" as the string concatenation
operator. Go figure!

|>ouglas

P.S. Overloading "left shift" to mean "output" does indeed seem a bit
sketchy, but in 15 years of C++ programming, I've never seen it cause
any confusion or bugs.

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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 12, 7:19 pm, Steven D'Aprano  wrote:

> You are making an unjustified assumption: \y is not an error.

You are making in an unjustified assumption that I ever made such an
assumption!

My claim is and has always been NOT that \y is inately an error, but
rather that treating unrecognized escape sequences as legal escape
sequences is error PRONE.

> While I'm amused that you've made my own point for me, I'm less
> amused that you seem to be totally incapable of seeing past your
> parochial language assumptions,

Where do you get the notion that my assumptions are in any sense
"parochial"? They come from (1) a great deal of experience programming
very reliable software, and (2) having learned at least two dozen
different programming languages in my life.

> I disagree with nearly everything you say in this post. I think
> that a few points you make have some validity, but the vast
> majority are based on a superficial and confused understanding
> of language design principles.

Whatever. I've taken two graduate level classes at MIT on programming
languages design, and got an A in both classes, and designed my own
programming language as a final project, and received an A+. But I
guess I don't really know anything about the topic at all.

> But it's not the only reasonable design choice, and Bash has
> made a different choice, and Python has made yet a third
> reasonable choice, and Pascal made yet a fourth reasonable choice.

And so did Perl and PHP, and whatever other programming language you
happen to mention. In fact, all programming languages are equally
good, so we might as well just freeze all language design as it is
now. Clearly we can do no better.

> One party insisting that red is the only logical colour for a
> car, and that anybody who prefers white or black or blue is
> illogical, is unacceptable.

If having all cars be red saved a lot of lives, or increased gas
mileage significantly, then it might very well be the best color for a
car. But of course, that is not the case. With programming languages,
there is much more likely to be an actual fact of the matter on which
sorts of language design decisions make programmers more productive on
average, and which ones result in more reliable software.

I will certainly admit that obtaining objective data on such things is
very difficult, but it's a completely different thing that one's color
preference for their car.

|>ouglas
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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 12, 5:32 am, Steven D'Aprano
 wrote:

> That problem basically boils down to a deep-seated
> philosophical disagreement over which philosophy a
> language should follow in regard to backslash escapes:
>
> "Anything not explicitly permitted is forbidden"
>
> versus
>
> "Anything not explicitly forbidden is permitted"

No, it doesn't. It boils down to whether a language should:

(1) Try it's best to detect errors as early as possible,
especially when the cost of doing so is low.

(2) Make code as readable as possible, in part by making
code as self-evident as possible by mere inspection and by
reducing the amount of stuff that you have to memorize. Perl
fails miserably in this regard, for instance.

(3) To quote Einstein, make everything as simple as
possible, and no simpler.

(4) Take innately ambiguous things and not force them to be
unambiguous by mere fiat.

Allowing a programmer to program using a completely
arbitrary resolution of "unrecognized escape sequences"
violates all of the above principles.

The fact that the meanings of unrecognized escape sequences
are ambiguous is proved by the fact that every language
seems to treat them somewhat differently, demonstrating that
there is no natural intuitive meaning for them.

Furthermore, allowing programmers to use "unrecognized escape
sequences" without raising an error violates:

(1) Explicit is better than implicit:

Python provides a way to explicitly specify that you want a
backslash. Every programmer should be encouraged to use
Python's explicit mechanism here.

(2) Simple is better than complex:

Python currently has two classes of ambiguously
interpretable escape sequences: "unrecognized ones", and
"illegal" ones. Making a single class (i.e. just illegal
ones) is simpler.

Also, not having to memorize escape sequences that you
rarely have need to use is simpler.

(3) Readability counts:

See above comments on readability.

(4) Errors should never pass silently:

Even the Python Reference Manual indicates that unrecognized
escape sequences are a source of bugs. (See more comments on
this below.)

(5) In the face of ambiguity, refuse the temptation to
guess.

Every language, other than C++, is taking a guess at what
the programmer would find to be most useful expansion for
unrecognized escape sequences, and each of the languages is
guessing differently. This temptation should be refused!

You can argue that once it is in the Reference Manual it is
no longer a guess, but that is patently specious, as Perl
proves. For instance, the fact that Perl will quietly convert
an array into a scalar for you, if you assign the array to a
scalar variable is certainly a "guess" of the sort that this
Python koan is referring to. Likewise for an arbitrary
interpretation of unrecognized escape sequences.

(6) There should be one-- and preferably only one --obvious
way to do it.

What is the one obvious way to express "\\y"? It is "\\y" or
"\y"?

Python can easily make one of these ways the "one obvious
way" by making the other one raise an error.

(7) Namespaces are one honking great idea -- let's do more
of those!

Allowing "\y" to self-expand is intruding into the namespace
for special characters that require an escape sequence.

> C++ apparently forbids all escape sequences, with
> unspecified behaviour if you use a forbidden sequence,
> except for a handful of explicitly permitted sequences.
>
> That's not better, it's merely different.

It *is* better, as it catches errors early on at little
cost, and for all the other reasons listed above.

> Actually, that's not true -- that the C++ standard forbids
> a thing, but leaves the consequences of doing that thing
> unspecified, is clearly a Bad Thing.

Indeed. But C++ has backward compatibly issues that make
any that Python has to deal with, pale in comparison. The
recommended behavior for a C++ compiler, however, is to flag
the problem as an error or as a warning.

> So on at least one machine in the world, C++ simply strips
> out backslashes that it doesn't recognize, leaving the
> suffix. Unfortunately, we can't rely on that, because C++
> is underspecified.

No, *fortunately* you can't rely on it, forcing you to go
fix your code.

> Fortunately this is not a problem with
> Python, which does completely specify the behaviour of
> escape sequences so there are no surprises.

It's not a surprise when the C++ compiler issues a warning to
you. If you ignore the warning, then you have no one to
blame but yourself.

> Implicit has an actual meaning. You shouldn't use it as a
> mere term of opprobrium for anything you don't like.

Pardon me, but I'm using "implicit" to mean "implicit", and
nothing more.

Python's behavior here is "implicit" in the very same way
that Perl implicitly converts an array into a scalar for
you. (Though that particular Perl behavior is a far bigger
wart than Python's behavior is here!)

> > Because you've stated that "\y" is a legal escape
> > sequence, while the Python Reference Manual

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 12, 3:36 am, Steven D'Aprano
 wrote:

> On Tue, 11 Aug 2009 13:20:52 -0700, Douglas Alan wrote:

> > My "Annotated C++ Reference Manual" is packed, and surprisingly in
> > Stroustrup's Third Edition, there is no mention of the issue in the
> > entire 1,000 pages. But Microsoft to the rescue:
>
> >      If you want a backslash character to appear within a string, you
> >      must type two backslashes (\\)
>
> > (From http://msdn.microsoft.com/en-us/library/69ze775t.aspx)
>
> Should I assume that Microsoft's C++ compiler treats it as an error, not
> a warning?

In my experience, C++ compilers generally generate warnings for such
situations, where they can. (Clearly, they often can't generate
warnings for running off the end of an array, which is also undefined,
though a really smart C++ compiler might be able to generate a warning
in certain such circumstances.)

> Or is is this *still* undefined behaviour, and MS C++ compiler
> will happily compile "ab\cd" whatever it feels like?

If it's a decent compiler, it will generate a warning. Who can say
with Microsoft, however. It's clearly documented as illegal code,
however.

> > The question of what any specific C++ does if you ignore the warning is
> > irrelevant, as such behavior in C++ is almost *always* undefined. Hence
> > the warning.
>
> So a C++ compiler which follows Python's behaviour would be behaving
> within the language specifications.

It might be, but there are also *recommendations* in the C++ standard
about what to do in such situations, and the recommendations say, I am
pretty sure, not to do that, unless the particular compiler in
question has to meet some very specific backward compatibility needs.

> I note that the bash shell, which claims to follow C semantics, also does
> what Python does:
>
> $ echo $'a s\trin\g with escapes'
> a s     rin\g with escapes

Really? Not on my computers. (One is a Mac, and the other is a Fedora
Core Linux box.) On my computers, bash doesn't seem to have *any*
escape sequences, other than \\, \", \$, and \`. It seems to treat
unknown escape sequences the same as Python does, but as there are
only four known escape sequences, and they are all meant merely to
guard against string interpolation, and the like, it's pretty darn
easy to keep straight.

> Explain to me again why we're treating underspecified C++ semantics,
> which may or may not do *exactly* what Python does, as if it were the One
> True Way of treating escape sequences?

I'm not saying that C++ does it right for Python. The right thing for
Python to do is to generate an error, as Python doesn't have to deal
with all the crazy complexities that C++ has to.

|>ouglas
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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 12, 3:08 am, Steven D'Aprano
 wrote:

> On Tue, 11 Aug 2009 14:48:24 -0700, Douglas Alan wrote:
> > In any case, my argument has consistently been that Python should have
> > treated undefined escape sequences consistently as fatal errors,
>
> A reasonable position to take. I disagree with it, but it is certainly
> reasonable.
>
> > not as warnings.
>
> I don't know what language you're talking about here, because non-special
> escape sequences in Python aren't either errors or warnings:
>
> >>> print "ab\cd"
>
> ab\cd

I was talking about C++, whose compilers tend to generate warnings for
this usage. I think that the C++ compilers I've used take the right
approach, only ideally they should be *even* more emphatic, and
elevate the problem from a warning to an error.

I assume, however, that the warning is a middle ground between doing
the completely right thing, and, I assume, maintaining backward
compatibility with common C implementations. As Python never had to
worry about backward compatibility with C, Python didn't have to walk
such a middle ground.

On the other hand, *now* it has to worry about backward compatibility
with itself.

|>ouglas


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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

I wrote:

> But you're right, it's too late to change this now.

P.S. But if it weren't too late, I think that your idea to have "\s"
be the escape sequence for a backslash instead of "\\" might be a good
one.

|>ouglas
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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 11, 4:38 pm, Ethan Furman  wrote:

> Mind you, I'm not really vested in how Python *should* handle
> backslashes one way or the other, but I am glad it has rules that it
> follows for consitent results, and I don't have to break out a byte-code
> editor to find out what's in my string literal.

I don't understand your comment. C++ generates a warning if you use an
undefined escape sequence, which indicates that your program should be
fixed. If the escape sequence isn't undefined, then C++ does the same
thing as Python.

It would be *even* better if C++ generated a fatal error in this
situation. (g++ probably has an option to make warnings fatal, but I
don't happen to know what that option is.) g++ might not generate an
error so that you can compile legacy C code with it.

In any case, my argument has consistently been that Python should have
treated undefined escape sequences consistently as fatal errors, not
as warnings.

|>ouglas

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http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 10, 11:27 pm, Steven D'Aprano
 wrote:
> On Mon, 10 Aug 2009 08:21:03 -0700, Douglas Alan wrote:
> > But you're right, it's too late to change this now.
>
> Not really. There is a procedure for making non-backwards compatible
> changes. If you care deeply enough about this, you could agitate for
> Python 3.2 to raise a PendingDepreciation warning for "unexpected" escape
> sequences like \z,

How does one do this?

Not that I necessarily think that it is important enough a nit to
break a lot of existing code.

Also, if I "agitate for change", then in the future people might
actually accurately accuse me of agitating for change, when typically
I just come here for a good argument, and I provide a connected series
of statements intended to establish a proposition, but in return I
receive merely the automatic gainsaying of any statement I make.

|>ouglas

-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

Steven D'Aprano wrote:

> Because the cost isn't zero. Needing to write \\ in a string
> literal when you want \ is a cost,

I need to preface this entire post with the fact that I've
already used ALL of the arguments that you've provided on my
friend before I ever even came here with the topic, and my
own arguments on why Python can be considered to be doing
the right thing on this issue didn't even convince ME, much
less him. When I can't even convince myself with an argument
I'm making, then you know there's a problem with it!

Now back the our regularly scheduled debate:

I think that the total cost of all of that extra typing for
all the Python programmers in the entire world is now
significantly less than the time it took to have this
debate. Which would have never happened if Python did things
the right way on this issue to begin with. Meaning that
we're now at LESS than zero cost for doing things right!

And we haven't even yet included all the useless heat that
is going to be generated during code reviews and in-house coding
standard debates.

That's why I stand by Python's motto:

   THERE SHOULD BE ONE-- AND PREFERABLY ONLY ONE --OBVIOUS
   WAY TO DO IT.

> and having to read \\ in source code and mentally
> translate that to \ is also a cost.

For me that has no mental cost. What does have a mental cost
is remembering whether "\b" is an "unrecognized escape
sequence" or not.

> By all means argue that it's a cost that is worth paying,
> but please stop pretending that it's not a cost.

I'm not "pretending". I'm pwning you with logic and common
sense!

> Having to remember that \n is a "special" escape and \y
> isn't is also a cost, but that's a cost you pay in C++ too,
> if you want your code to compile.

Ummm, no I don't! I just always use "\\" when I want a
backslash to appear, and I only think about the more obscure
escape sequences if I actually need them, or some code that
I am reading has used them.

> By the way, you've stated repeatedly that \y will compile
> with a warning in g++. So what precisely do you get if you
> ignore the warning?

A program with undefined behavior. That's typically what a
warning means from a C++ compiler. (Sometimes it means
use of a deprecated feature, though.)

> What do other C++ compilers do?

The Microsoft compilers also consider it to be incorrect
code, as I documented in a different post.

> Apart from the lack of warning, what actually is the
> difference between Python's behavior and C++'s behavior?

That question makes just about as much sense as, "Apart
from the lack of a fatal error, what actually is the
difference between Python's behavior and C++'s?"

Sure, warnings aren't fatal errors, but if you ignore them,
then you are almost always doing something very
wrong. (Unless you're building legacy code.)

> > Furthermore, Python's strategy here is SPECIFICALLY
> > DESIGNED, according to the reference manual to catch
> > bugs. I.e., from the original posting on this issue:
>
> >  Unlike Standard C, all unrecognized escape sequences
> >  are left in the string unchanged, i.e., the backslash
> >  is left in the string.  (This behavior is useful when
> >  debugging: if an escape sequence is mistyped, the
> >  resulting output is more easily recognized as
> >  broken.)
>
> You need to work on your reading comprehension. It doesn't
> say anything about the motivation for this behaviour, let
> alone that it was "SPECIFICALLY DESIGNED" to catch bugs. It
> says it is useful for debugging. My shoe is useful for
> squashing poisonous spiders, but it wasn't designed as a
> poisonous-spider squashing device.

As I have a BS from MIT in BS-ology, I can readily set aside
your aspersions to my intellect, and point out the gross
errors of your ways: Natural language does not work the way
you claim. It is is much more practical, implicit, and
elliptical.

More specifically, if your shoe came with a reference manual
claiming that it was useful for squashing poisonous spiders,
then you may now validly assume poisonous spider squashing
was a design requirement of the shoe. (Or at least it has
become one, even if ipso facto.) Furthermore, if it turns out
that the shoe is deficient at poisonous spider squashing,
and consequently causes you to get bitten by a poisonous
spider, then you now have grounds for a lawsuit.

> > Because in the former cases it can't catch the the bug,
> > and in the latter case, it can.
>
> I'm not convinced this is a bug that needs catching, but if
> you think it is, then that's a reasonable argument.

All my arguments are reasonable.

> >> Perhaps it can catch *some* errors of that type, but
> >> only at the cost of extra effort required to defeat the
> >> compiler (forcing the programmer to type \\d to prevent
> >> the compiler complaining about \d). I don't think the
> >> benefit is worth the cost. You and your friend do. Who
> >> is to say you're right?
>
> > Well, Bjarne Stroustrup, for one.
>
> Then let him design hi

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 11, 2:00 pm, Steven D'Aprano  wrote:

> > test.cpp:1:1: warning: unknown escape sequence '\y'
>
> Isn't that a warning, not a fatal error? So what does temp contain?

My "Annotated C++ Reference Manual" is packed, and surprisingly in
Stroustrup's Third Edition, there is no mention of the issue in the
entire 1,000 pages. But Microsoft to the rescue:

 If you want a backslash character to appear within a string,
 you must type two backslashes (\\)

(From http://msdn.microsoft.com/en-us/library/69ze775t.aspx)

The question of what any specific C++ does if you ignore the warning
is irrelevant, as such behavior in C++ is almost *always* undefined.
Hence the warning.

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

From: Steven D'Aprano  wrote:

> On Mon, 10 Aug 2009 00:32:30 -0700, Douglas Alan wrote:

> > In C++, if I know that the code I'm looking at compiles,
> > then I never need worry that I've misinterpreted what a
> > string literal means.

> If you don't know what your string literals are, you don't
> know what your program does. You can't expect the compiler
> to save you from semantic errors. Adding escape codes into
> the string literal doesn't change this basic truth.

I grow weary of these semantic debates. The bottom line is
that C++'s strategy here catches bugs early on that Python's
approach doesn't. It does so at no additional cost.

>From a purely practical point of view, why would any
language not want to adopt a zero-cost approach to catching
bugs, even if they are relatively rare, as early as
possible?

(Other than the reason that adopting it *now* is sadly too
late.)

Furthermore, Python's strategy here is SPECIFICALLY
DESIGNED, according to the reference manual to catch bugs.
I.e., from the original posting on this issue:

 Unlike Standard C, all unrecognized escape sequences
 are left in the string unchanged, i.e., the backslash
 is left in the string. (This behavior is useful when
 debugging: if an escape sequence is mistyped, the
 resulting output is more easily recognized as broken.)

If this "feature" is designed to catch bugs, why be
half-assed about it? Especially since there seems to be
little valid use case for allowing programmers to be lazy in
their typing here.

> The compiler can't save you from typing 1234 instead of
> 11234, or 31.45 instead of 3.145, or "My darling Ho"
> instead of "My darling Jo", so why do you expect it to
> save you from typing "abc\d" instead of "abc\\d"?

Because in the former cases it can't catch the the bug, and
in the latter case, it can.

> Perhaps it can catch *some* errors of that type, but only
> at the cost of extra effort required to defeat the
> compiler (forcing the programmer to type \\d to prevent
> the compiler complaining about \d). I don't think the
> benefit is worth the cost. You and your friend do. Who is
> to say you're right?

Well, Bjarne Stroustrup, for one.

All of these are value judgments, of course, but I truly
doubt that anyone would have been bothered if Python from
day one had behaved the way that C++ does. Additionally, I
expect that if Python had always behaved the way that C++
does, and then today someone came along and proposed the
behavior that Python currently implements, so that the
programmer could sometimes get away with typing a bit less,
such a person would be chided for not understanding the Zen
of Python.

> > You don't have to go running for the manual every time
> > you see code with backslashes, where the upshot might be
> > that the programmer was merely saving themselves some
> > typing.

> Why do you care if there are "funny characters"?

Because, of course, "funny characters" often have
interesting consequences when output. Furthermore, their
consequences aren't always immediately obvious from looking
at the source code, unless you are intimately familiar with
the function of the special characters in question.

For instance, sometimes in the wrong combination, they wedge
your xterm. Etc.

I'm surprised that this needs to be spelled out.

> In C++, if you see an escape you don't recognize, do you
> care?

Yes, of course I do. If I need to know what the program
does.

> Do you go running for the manual? If the answer is No,
> then why do it in Python?

The answer is that I do in both cases.

> No. \z *is* a legal escape sequence, it just happens to map to \z.

> If you stop thinking of \z as an illegal escape sequence
> that Python refuses to raise an error for, the problem
> goes away. It's a legal escape sequence that maps to
> backslash + z.

(1) I already used that argument on my friend, and he wasn't
buying it. (Personally, I find the argument technically
valid, but commonsensically invalid. It's a language-lawyer
kind of argument, rather than one that appeals to any notion
of real aesthetics.)

(2) That argument disagrees with the Python reference
manual, which explicitly states that "unrecognized escape
sequences are left in the string unchanged", and that the
purpose for doing so is because it "is useful when
debugging".

> > "\x" is not a legal escape sequence. Shouldn't it also
> > get left as "\\x"?
>
> No, because it actually is an illegal escape sequence.

What makes it "illegal". As far as I can tell, it's just
another "unrecognized escape sequence". JavaScript treats it
that way. Are you goin

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 10, 10:58 am, Scott David Daniels 
wrote:

> The string rules reflect C's rules, and I see little
> excuse for trying to change them now.

No they don't. Or at least not C++'s rules. C++ behaves exactly as I
should like.

(Or at least g++ does. Or rather *almost* as I would like, as by
default it generates a warning for "foo\zbar", while I think that an
error would be somewhat preferable.)

But you're right, it's too late to change this now.

|>ouglas

-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 10, 4:37 am, Steven D'Aprano

> There is at least one good reason for preferring an error, namely that it
> allows Python to introduce new escape codes without going through a long,
> slow process. But the rest of these complaints are terribly unconvincing.


What about:

   o Beautiful is better than ugly
   o Explicit is better than implicit
   o Simple is better than complex
   o Readability counts
   o Special cases aren't special enough to break the rules
   o Errors should never pass silently

?

And most importantly:

   o In the face of ambiguity, refuse the temptation to guess.
   o There should be one -- and preferably only one -- obvious way to
do it.

?

So, what's the one obvious right way to express "foo\zbar"? Is it

   "foo\zbar"

or

   "foo\\zbar"

And if it's the latter, what possible benefit is there in allowing the
former?  And if it's the former, why does Python echo the latter?

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 10, 2:10 am, Steven D'Aprano

> I've never had any errors caused by this.

But you've seen an error caused by this, in this very discussion.
I.e., "foo\xbar".

"\xba" isn't an escape sequence in any other language that I've used,
which is one reason I made this error... Oh, wait a minute -- it *is*
an escape sequence in JavaScript. But in JavaScript, while "\xba" is a
special character, "\xb" is synonymous with "xb".

The fact that every language seems to treat these things similarly but
differently, is yet another reason why they should just be treated
utterly consistently by all of the languages: I.e., escape sequences
that don't have a special meaning should be an error!

> I've never seen anyone write to
> this newsgroup confused over escape behaviour,

My friend objects strongly the claim that he is "confused" by it, so I
guess you are right that no one is confused. He just thinks that it
violates the beautiful sense of aesthetics that he was sworn over and
over again Python to have.

But aesthetics is a non-negligible issue with practical ramifications.
(Not that anything can be done about this wart at this point,
however.)

> or asking for help with an error caused by it, and until
> this thread, never seen anyone complain about it either.

Oh, this bothered me too when I first learned Python, and I thought it
was stupid. It just didn't bother me enough to complain publicly.

Besides, the vast majority of Python noobs don't come here, despite
appearance sometimes, and by the time most people get here, they've
probably got bigger fish to fry.

|>ouglas


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Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 10, 2:03 am, Steven D'Aprano
 wrote:

> On Sun, 09 Aug 2009 17:56:55 -0700, Douglas Alan wrote:

> > Because in Python, if my friend sees the string "foo\xbar\n", he has no
> > idea whether the "\x" is an escape sequence, or if it is just the
> > characters "\x", unless he looks it up in the manual, or tries it out in
> > the REPL, or what have you.
>
> Fair enough, but isn't that just another way of saying that if you look
> at a piece of code and don't know what it does, you don't know what it
> does unless you look it up or try it out?

Not really. It's more like saying that easy things should be easy, and
hard things should possible. But in this case, Python is making
something that should be really easy, a bit harder and more error
prone than it should be.

In C++, if I know that the code I'm looking at compiles, then I never
need worry that I've misinterpreted what a string literal means. At
least not if it doesn't have any escape characters in it that I'm not
familiar with. But in Python, if I see, "\f\o\o\b\a\z", I'm not really
sure what I'm seeing, as I surely don't have committed to memory some
of the more obscure escape sequences. If I saw this in C++, and I knew
that it was in code that compiled, then I'd at least know that there
are some strange escape codes that I have to look up. Unlike with
Python, it would never be the case in C++ code that the programmer who
wrote the code was just too lazy to type in "\\f\\o\\o\\b\\a\\z"
instead.

> > My friend is adamant that it would be better
> > if he could just look at the string literal and know. He doesn't want to
> > be bothered to have to store stuff like that in his head. He wants to be
> > able to figure out programs just by looking at them, to the maximum
> > degree that that is feasible.
>
> I actually sympathize strongly with that attitude. But, honestly, your
> friend is a programmer (or at least pretends to be one *wink*).

Actually, he's probably written more code than you, me, and ten other
random decent programmers put together. As he can slap out massive
amounts of code very quickly, he'd prefer not to have crap getting in
his way. In the time it takes him to look something up, he might have
written another page of code.

He's perfectly capable of dealing with crap, as years of writing large
programs in Perl and PHP quickly proves, but his whole reason for
learning Python, I take it, is so that he will be bothered with less
crap and therefore write code even faster.

> You can't be a programmer without memorizing stuff: syntax, function
> calls, modules to import, quoting rules, blah blah blah. Take C as
> an example -- there's absolutely nothing about () that says "group
> expressions or call a function" and {} that says "group a code
> block".

I don't really think that this is a good analogy. It's like the
difference between remembering rules of grammar and remembering
English spelling. As a kid, I was the best in my school at grammar,
and one of the worst at speling.

> You just have to memorize it. If you don't know what a backslash
> escape is going to do, why would you use it?

(1) You're looking at code that someone else wrote, or (2) you forget
to type "\\" instead of "\" in your code (or get lazy sometimes), as
that is okay most of the time, and you inadvertently get a subtle bug.

> This is especially important when reading (as opposed to writing) code.
> You read somebody else's code, and see "foo\xbar\n". Let's say you know
> it compiles without warning. Big deal -- you don't know what the escape
> codes do unless you've memorized them. What does \n resolve to? chr(13)
> or chr(97) or chr(0)? Who knows?

It *is* a big deal. Or at least a non-trivial deal. It means that you
can tell just by looking at the code that there are funny characters
in the string, and not just a backslashes. You don't have to go
running for the manual every time you see code with backslashes, where
the upshot might be that the programmer was merely saving themselves
some typing.

> > In comparison to Python, in C++, he can just look "foo\xbar\n" and know
> > that "\x" is a special character. (As long as it compiles without
> > warnings under g++.)
>
> So what you mean is, he can just look at "foo\xbar\n" AND COMPILE IT
> USING g++, and know whether or not \x is a special character.

I'm not sure that your comments are paying due diligence to full
life-cycle software development issues that involve multiple
programmers (or even just your own program that you wrote a year ago,
and you don't remember all the details of wh

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

On Aug 9, 8:06 pm, Steven D'Aprano wrote:

> while the behaviour your
> friend wants is "treat a backslash as an error, except for these
> exceptions".

Besides, can't all error situations be described as, "treat the error
situation as an error, except for the exception of when the situation
isn't an error"???

The behavior my friend wants isn't any more exceptional than that!

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Unrecognized escape sequences in string literals

[Douglas Alan]

Steven D'Aprano wrote:

> Why should a backslash in a string literal be an error?

Because in Python, if my friend sees the string "foo\xbar\n", he has
no idea whether the "\x" is an escape sequence, or if it is just the
characters "\x", unless he looks it up in the manual, or tries it out
in the REPL, or what have you. My friend is adamant that it would be
better if he could just look at the string literal and know. He
doesn't want to be bothered to have to store stuff like that in his
head. He wants to be able to figure out programs just by looking at
them, to the maximum degree that that is feasible.

In comparison to Python, in C++, he can just look "foo\xbar\n" and
know that "\x" is a special character. (As long as it compiles without
warnings under g++.)

He's particularly annoyed too, that if he types "foo\xbar" at the
REPL, it echoes back as "foo\\xbar". He finds that to be some sort of
annoying DWIM feature, and if Python is going to have DWIM features,
then it should, for example, figure out what he means by "\" and not
bother him with a syntax error in that case.

Another reason that Python should not behave the way that it does, is
that it pegs Python into a corner where it can't add new escape
sequences in the future, as doing so will break existing code.
Generating a syntax error instead for unknown escape sequences would
allow for future extensions.

Now not to pick on Python unfairly, most other languages have similar
issues with escape sequences. (Except for the Bourne Shell and bash,
where "\x" always just means "x", no matter what character "x" happens
to be.) But I've been telling my friend for years to switch to Python
because of how wonderful and consistent Python is in comparison to
most other languages, and now he seems disappointed and seems to think
that Python is just more of the same.

Of course I think that he's overreacting a bit. My point of view is
that every language has *some* warts; Python just has a bit fewer than
most. It would have been nice, I should think, if this wart had been
"fixed" in Python 3, as I do consider it to be a minor wart.

|>ouglas

-- 
http://mail.python.org/mailman/listinfo/python-list

Unrecognized escape sequences in string literals

[Douglas Alan]

A friend of mine is just learning Python, and he's a bit tweaked about
how unrecognized escape sequences are treated in Python. This is from
the Python 3.0 reference manual:

 Unlike Standard C, all unrecognized escape sequences are left in
the string unchanged, i.e.,
 the backslash is left in the string. (This behavior is useful
when debugging: if an escape
 sequence is mistyped, the resulting output is more easily
recognized as broken.) It is also
 important to note that the escape sequences only recognized in
string literals fall into the
 category of unrecognized escapes for bytes literals.

My friend begs to differ with the above. It would be much better for
debugging if Python generated a parsing error for unrecognized escape
sequences, rather than leaving them unchanged. g++ outputs a warning
for such escape sequences, for instance. This is what I would consider
to be the correct behavior. (Actually, I think it should just generate
a fatal parsing error, but a warning is okay too.)

In any case, I think my friend should mellow out a bit, but we both
consider this something of a wart. He's just more wart-phobic than I
am. Is there any way that this behavior can be considered anything
other than a wart? Other than the unconvincing claim that you can use
this "feature" to save you a bit of typing sometimes when you actually
want a backslash to be in your string?

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Efficient binary search tree stored in a flat array?

[Douglas Alan]

I wrote:

> On Jul 14, 8:10 am, Piet van Oostrum  wrote:
>
> > Of course you can take any BST algorithm and replace pointers by indices
> > in the array and allocate new elements in the array. But then you need
> > array elements to contain the indices for the children explicitely.


> And why is this a problem?

Oh, I'm sorry -- I see what you are saying now. You're saying you can
just implement a normal binary search tree, but store the tree nodes
in an array, as if it were a chunk of memory, and use array indices as
pointers, rather than using memory addresses as pointers.

Fair enough, but I'm not sure what that would buy you. Other than,
perhaps some improved locality of reference, and the potential to
perhaps get the pointers take up less space if you know the array is
never going to grow to be very large.

|>ouglas

-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Efficient binary search tree stored in a flat array?

[Douglas Alan]

On Jul 14, 9:19 am, Scott David Daniels  wrote:

> It may well be that there is no good simple solution, and people avoid
> writing about non-existent algorithms.

I can't imagine why that should be the case. The CLRS textbook on
algorithms, for instance, goes to some pains to mathematically prove
that there is no comparison sort that can operate in faster than O(n
log n) time.

And any decent discussion of rabies would be sure to mention that
there is no known cure.

CLRS talks about binary heaps, binary search trees, and treaps, and it
shows how to maintain a binary heap in a flat array efficiently (n log
n time overall), but it never even seems to bring up the subject as to
whether a binary search tree or a treap can also be efficiently
maintained in a flat array.

Though it may be the case that these questions are left as exercises
for the student, and therefore buried in the exercises and problem
sets that I didn't read carefully.

> Piet van Oostrum wrote:

> > Of course you can take any BST algorithm and replace pointers by indices
> > in the array and allocate new elements in the array. But then you need
> > array elements to contain the indices for the children explicitely.

> And you loower your locality of reference (cache-friendliness).
> Note the insert in Python, for example, is quite cache-friendly.

I can't see that a binary search tree would typically have
particularly good cache-friendliness, so I can't see why a flat-array
representation, such as is done for a binary heap, would have
particularly worse cache-reference.

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Efficient binary search tree stored in a flat array?

[Douglas Alan]

On Jul 14, 8:10 am, Piet van Oostrum  wrote:

> Of course you can take any BST algorithm and replace pointers by indices
> in the array and allocate new elements in the array. But then you need
> array elements to contain the indices for the children explicitely.

And why is this a problem? This is how binary heaps are typically
implemented, and it all works swimmingly. The node rotations for
keeping a binary heap balanced turn out to be suitable for
representation in a flat array. I.e., when you do the node rotations,
you only ever have to copy log n array elements.

In general, however, you can't move nodes around so easily when
represented in a flat array. A node movement in a tree represented
with pointers, might involves changing just two pointers, while when
represented as a flat array, might involve copying most of the array
to maintain the tree invariants. It just so turns out that maintaining
the invariants for a binary heap does not have this issue.

This is why I was curious about treaps, which are a type of binary
heap. The CLRS textbook on algorithms discusses treaps, but doesn't
ever mention whether they are as fortunate as less constrained binary
heaps. I'm sure I could work out for myself whether the treap
rotations are suitable for storage in a flat array, but I might make a
mistake somewhere in my reasoning, and then never know the true
answer!

|>ouglas
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Efficient binary search tree stored in a flat array?

[Douglas Alan]

On Jul 14, 7:38 am, Florian Brucker  wrote:

> Douglas Alan wrote:

> > Thank you. My question wasn't intended to be Python specific, though.
> > I am just curious for purely academic reasons about whether there is
> > such an algorithm. All the sources I've skimmed only seem to the
> > answer the question via omission. Which is kind of strange, since it
> > seems to me like an obvious question to ask.

> IIRC comp.programming would be the place to ask such questions.

Thanks, yes that does look like a good place to post such questions.
Unfortunately, it also looks to be overrun with stories on "hot girls
top and bottom sexy movie", though I'm sure I can ignore those. I'm
scared to look at the posting on "tricky bit twiddling", though.

|>ouglas

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http://mail.python.org/mailman/listinfo/python-list

Re: Efficient binary search tree stored in a flat array?

[Douglas Alan]

On Jul 13, 3:57 pm, a...@pythoncraft.com (Aahz) wrote:

> Still, unless your list is large (more than thousands of elements),
> that's the way you should go.  See the bisect module.  Thing is, the
> speed difference between C and Python means the constant for insertion
> and deletion is very very small relative to bytecode speed.  Keep in
> mind that Python's object/binding model means that you're shuffling
> pointers in the list rather than items.

Thank you. My question wasn't intended to be Python specific, though.
I am just curious for purely academic reasons about whether there is
such an algorithm. All the sources I've skimmed only seem to the
answer the question via omission. Which is kind of strange, since it
seems to me like an obvious question to ask.

If I find the free time, I might try to work out myself whether it can
be done with a treap.

|>ouglas
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Efficient binary search tree stored in a flat array?

[Douglas Alan]

I couldn't find a good algorithms forum on the Internet, so I guess
I'll ask this question here instead: Is it possible to efficiently
maintain a binary search tree in a flat array (i.e., without using
pointers), as is typically done for a binary heap?

It *is* possible, of course, to keep an ordered list in a flat array,
and then do a binary search on the ordered array, but then insertion
and deletion are O(n), rather than O(log n). It would also clearly be
possible to store a balanced binary tree in a flat array, storing the
children of the node at index i at indices 2*i and 2*i + 1, just as
one does for a binary heap. But if you do that, I don't know if you
could then do insertions and deletions in O(log n) time.

One idea that came to mind, is that maybe it is possible using a
"treap", which is a combination of a binary heap and a binary search
tree. Insertions and deletions in binary heaps can be done in O(log n)
in a flat array, but I don't know if this is also true for a treap,
since you also have the binary search tree invariants to maintain, in
addition to the binary heap invariants. For all I know, this might
cause rotations to no longer be O(log n).

|>ouglas
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Re: Finding the instance reference of an object

[Douglas Alan]

Joe Strout <[EMAIL PROTECTED]> writes:

>>   Q. What type of calling semantics do Python and Java use?
>>
>>   A. Call-by-sharing.
>
> Fair enough, but if the questioner then says "WTF is call-by-sharing,"
> we should answer "call-by-sharing is the term we prefer for call-by-
> value in the case where the value is an object reference (as is always
> the case in Python)."

Personally, I think that it is much preferable to leave
"call-by-value" completely out of any such discussion, as it provably
leads to a great deal of confusion and endless, pointless debate.
It's better to just start from a clean slate and explain how
call-by-sharing works, and to assert that it is quite different from
the calling semantics of languages such as C or Pascal or Fortran, so
the student must set aside any preconceptions about how argument
passing works.

Call-by-sharing is technically a type of call-by-value only for those
who are devotees of academic programming language zoology.  For
everyone else, call-by-sharing is its own beast.  One might point all
of this out in the discussion, however, if it will help the other
person understand.  You never know -- they might be a fan of academic
programming zoology.

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

greg <[EMAIL PROTECTED]> writes:

> Steven D'Aprano wrote:

>> At least some sections of the Java community seem to prefer a
>> misleading and confusing use of the word "value" over clarity and
>> simplicity, but I for one do not agree with them.

> I don't see anything inherently confusing or misleading
> about it. Confusion only arises when some people jump up
> and say that it's wrong to use the terms that way, because
> it might cause confusion...

Personally, I find this whole debate kind of silly, as it is based on
a completely fallacious either/or dichotomy.

(1) It is unarguably true that Python and Java use a type of
call-by-value.  This follows from the standard definition of
call-by-value, and common usage in, for example, the Scheme and
Java communities, etc.

(2) It is also unarguably true that saying that Python or Java use
"call-by-value", and saying nothing more is going to be profoundly
confusing to anyone who is learning these languages.

It's like the difference between

   Q. What is a giraffe?

   A. A giraffe is a type of animal.

and

   Q. What is Greg?

   A. Greg is a type of animal.

In both cases, the answers are strictly correct, but in the second
case, the answer is also deeply misleading.

Q. How do we generally solve this problem when speaking?

A. We invent more specific terms and then generally stick to the more
specific terms when the more general terms would be misleading.

I.e.,

   Q. What is Greg?

   A. Greg is a human being.

and

   Q. What type of calling semantics do Python and Java use?

   A. Call-by-sharing.

I assert that anyone who does not understand all of the above, is
helping to spread confusion.

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

Joe Strout <[EMAIL PROTECTED]> writes:

> Yes, OK, that's great.  But there are several standard pass-by-
> somethings that are defined by the CS community, and which are simple
> and clear and apply to a wide variety of languages.  "Pass by object"
> isn't one of them.

"Call-by-sharing" *is* one of them, and the term has been around since
the 70s:

   http://hopl.murdoch.edu.au/showlanguage.prx?exp=637

> I guess if you want to campaign for it as a shorthand for "object
> reference passed by value," you could do that, and it's not
> outrageous.

There's no need for a campaign.  The term has already been used in the
academic literature for 34 years.

> But to anybody new to the term, you should explain it as exactly
> that, rather than try to claim that Python is somehow different from
> other OOP languages where everybody calls it simply pass by value.

It's not true that "everybody calls it simply pass by value".

> OK, if there were such a thing as "pass-by-object" in the standard
> lexicon of evaluation strategies, I would be perfectly happy saying
> that a system has it if it behaves as though it has it, regardless of
> the underpinnings.

There is "call-by-sharing" in the standard lexicon of evaluation
strategies, and it's been in the lexicon since 1974.

> However, if you really think the term is that handy, and we want to
> agree to say "Python uses pass by object" and answer the inevitable
> "huh?" question with "that's shorthand for object references passed by
> value," then I'd be OK with that.

Excellent.  We can all agree to get along then!

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

Joe Strout <[EMAIL PROTECTED]> writes:

> As for where I get my definitions from, I draw from several sources:
>
> 1. Dead-tree textbooks

You've been reading the wrong textbooks.  Read Liskov -- she's called
CLU (and hence Python's) calling strategy "call-by-sharing" since the
70s.

> 2. Wikipedia [2] (and yes, I know that has to be taken with a grain of
> salt, but it's so darned convenient)
> 3. My wife, who is a computer science professor and does compiler
> research
> 4. http://javadude.com/articles/passbyvalue.htm (a brief but excellent
> article)
> 5. Observations of the "ByVal" (default) mode in RB and VB.NET
> 6. My own experience implementing the RB compiler (not that
> implementation details matter, but it forced me to think very
> carefully about references and parameter passing for a very long time)
>
> Not that I'm trying to argue from authority; I'm trying to argue from
> logic.  I suspect, though, that your last comment gets to the crux of
> the matter, and reinforces my guess above: you don't think c-b-v means
> what most people think it means.  Indeed, you don't think any of the
> languages shown at [1] are, in fact, c-b-v languages.  If so, then we
> should focus on that and see if we can find a definitive answer.

I'll give you the definitive answer from a position of authority,
then.  I took Barbara Liskov's graduate-level class in programming
language theory at MIT, and she called what Python does
"call-by-sharing".

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

greg <[EMAIL PROTECTED]> writes:

> Douglas Alan wrote:
>> greg <[EMAIL PROTECTED]> writes:
>
>>> This holds for *all* languages that I know about, both static and
>>> dynamic.
>
>> Then you don't know about all that many languages.  There are
>> languages that use call-by-name, and those that use
>> call-by-value-return.  Some use call-by-need and others do
>> call-by-macro-expansion.  Etc.
>
> I didn't mean that these are the only two parameter passing
> mechanisms in existence -- I know there are others.

I don't follow you.  You stated that once you understand how
assignment works, you understand the calling mechanism.  That's just
not true.  Algol, for instance, did assignment-by-value but
call-by-name.

>> If I tell you, for instance, that Java, Python, Ruby, JavaScript,
>> Lisp, and CLU all use call-by-sharing, then I have said something that
>> makes a similarity among these languages easier to state and easier to
>> grasp.
>
> If you told me they use "assignment by sharing", that would tell me
> a lot *more* about the language than just talking about parameter
> passing.

Not really.  Call-by-sharing virtually implies that the language does
assignment-by-sharing.  (I know of no counter-examples, and it is
difficult to see how a violation of this rule-of-thumb would be useful
in any new language.)  Stating that a language does
assignment-by-sharing does not imply that it does call-by-sharing.  Or
at least not exclusively so.  Cf. certain dialects of Lisp.  Also C#,
which supports a variety of argument passing strategies.

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

greg <[EMAIL PROTECTED]> writes:

> Douglas Alan wrote:
>> greg <[EMAIL PROTECTED]> writes:
>>
>>>Seems to me that (1) describes exactly how parameter passing
>>>works in Python. So why insist that it's *not* call by value?
>> Because there's an important distinction to be made,
>
> The distinction isn't about parameter passing, though, it's about
> the semantics of *assignment*. Once you understand how assigment
> works in Python, all you need to know then is that parameters are
> passed by assigning the actual parameter to the formal
> parameter. All else follows from that.
>
> This holds for *all* languages that I know about, both
> static and dynamic.

Then you don't know about all that many languages.  There are
languages that use call-by-name, and those that use
call-by-value-return.  Some use call-by-need and others do
call-by-macro-expansion.  Etc.  These languages generally don't use
these same semantics for assignment.

All languages that I know of that use call-by-sharing also do
assignment-by-sharing.  Not all languages that do
assignment-by-sharing always do only call-by-sharing, however.  For
instance, most dialects of Lisp have procedural macros.  The calling
semantics of procedural macros are quite different from the calling
semantics of normal functions, even though procedural macros are Lisp
functions.  Other dialects of Lisp provide the ability to state that
certain function arguments won't be evaluated at call time.  All
dialects of Lisp, however, do assignment-by-sharing, or "binding" as
it is called in the Lisp community.

Also, one could certainly invent additional languages that do behave
in the typical manners.

If you are merely asserting, however, that understanding how Python
does assignment will help you understand how Python does argument
passing, then you are certainly correct.  This, however, does not
imply that there is not a pre-existing precise terminology to describe
Python's calling semantics, and that this term can be useful in
describing how Python works.

If I tell you, for instance, that Java, Python, Ruby, JavaScript,
Lisp, and CLU all use call-by-sharing, then I have said something that
makes a similarity among these languages easier to state and easier to
grasp.

> Once you know how assignment works in the language concerned, then
> you know how parameter passing works as well. There is no need for
> new terms.

This is simply not true.

>> and the distinction has been written up in the Computer Science
>> literature since Lisp first starting using the same argument
>> passing semantics as Python back in 1958.  The semantics are called
>> "call by sharing".
>
> I still think it's an unnecessary term, resulting from confusion on
> the part of the authors about the meanings of the existing terms.

Trust me, Barbara Liskov was not one bit confused when she invented
the term "call-by-sharing". And her language CLU was one of the most
prescient to have ever been designed and implmented.

> If there's any need for a new term, it would be "assignment by
> sharing". Although there's already a term in use for that, too --
> it's known as reference assignment.

Reference assignement doesn't imply that the object is allocated on a
heap, and "call-by-sharing" does.

>> Many mainstream programming languages other than Python now use call
>> by sharing.  They include Java, JavaScript, Ruby, ActionScript, and C#.
>
> I would say they use assignment by sharing, and call by value.

We can also argue about how many angels can dance on the head of a
pin, but the term "call-by-sharing" has been around since the 70s, and
it is already well-defined and well understood.

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

Joe Strout <[EMAIL PROTECTED]> writes:

> There are only the two cases, which Greg quite succinctly and
> accurately described above.  One is by value, the other is by
> reference.  Python quite clearly uses by value.

You make a grave error in asserting that there are only two cases.
Algol, for instance, used call-by-name, which is neither call-by-value
or call-by-reference.  There are a number of other evaluation
strategies.  For a primer on the subject see the following Wikipedia
page:

   http://en.wikipedia.org/wiki/Evaluation_strategy

CLU used the termed "call-by-sharing" for the evaluation strategy
shared by Python, Lisp, CLU, Java, Ruby, and JavaScript, etc.

It should be noted that the Wikipedia page does not document
"call-by-sharing", in specific and refers to Python's strategy as a
type of call-by-value.  It also notes that call-by-value is not a
single evaluation strategy, but rather a family of evaluation
strategies, and that the version of the strategy used by Java (and
hence Python) shares features with call-by-reference strategies.

Consequently, many people prefer to use a different name from
"call-by-value" for Python/Java/Lisp's strategy in order to avoid
confusion.  In any case, no one can disagree with the fact that the
evaluation strategy used by Python et. al., differs significantly from
the call-by-value evaluation strategy used by C and the like, whatever
you wish to call it.

|>oug
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Re: Finding the instance reference of an object

[Douglas Alan]

greg <[EMAIL PROTECTED]> writes:

> Seems to me that (1) describes exactly how parameter passing
> works in Python. So why insist that it's *not* call by value?

Because there's an important distinction to be made, and the
distinction has been written up in the Computer Science literature
since Lisp first starting using the same argument passing semantics as
Python back in 1958.  The semantics are called "call by sharing".

Many mainstream programming languages other than Python now use call
by sharing.  They include Java, JavaScript, Ruby, ActionScript, and C#.

|>oug

P.S. Lisp didn't call it "call by sharing" -- Lisp called it
"binding".  The designers of CLU invented the term "call by sharing"
back in the 70s.  (Or at least I believe they invented the term.  They
certainly did use the term.)
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Re: Finding the instance reference of an object

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> I understand that Python's object and calling semantics are exactly the 
> same as Emerald (and likely other languages as well), and that both 
> Emerald and Python are explicitly based on those of CLU, as described by 
> by Barbara Liskov in 1979:
>
> "In particular it is not call by value because mutations of
> arguments performed by the called routine will be visible to the
> caller. And it is not call by reference because access is not
> given to the variables of the caller, but merely to certain
> objects."

It is quite true that Python's calling semantics are the same as
CLU's, and that CLU called these semantics "call by sharing".  It's
not quite true that CLU invented these calling semantics, however.
They were the calling semantics of Lisp long before CLU existed.  I'm
not sure that Lisp had a name for it, however.  Lisp people tended to
refer to all variable assignment as "binding".

I agree with those who object to calling the Python/CLU/Lisp calling
semantics as either "call by value" or "call by reference", which is
why I've been a bit dismayed over the years that the term "call by
sharing" hasn't caught on.  When this terminology is used, the meaning is
quite unambiguous.

It should be noted that many other mainstream languages now use call
by sharing.  It's not at all the least peculiar to Python.  Such
languages include Java, JavaScript, Ruby, C#, and ActionScript.

|>oug
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Re: The best platform and editor for Python

[Douglas Alan]

[EMAIL PROTECTED] (Alex Martelli) writes:

> Kay Schluehr <[EMAIL PROTECTED]> wrote:

>> half of the community is happy with Emacs and the other half wants to
>> program in a VS-like environment, neither consensus nor progress has

> Calling all vi/vim users (and we'll heartily appreciate the support
> of TextMate fans, BBEdit ones, etc, etc) -- we're at risk being
> defined out of existence, since we're neither happy with Emacs

That's because the Emacs users are the only ones who matter.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steve Holden <[EMAIL PROTECTED]> writes:

>> I'm relying on a feature that has worked fine since the early '90s,
>> and if it is ever changed in the future, I'm sure that plenty of other
>> language changes will come along with it that will make adapting code
>> that relies on this feature to be the least of my porting worries.

> Damn, it seems to be broken on my Jython/IronPython installations,
> maybe I should complain. Oh no, I can't, because it *isn't* *part*
> *of* *the* *language*. ...

As I have mentioned *many* times, I'm coding in CPython 2.5, and I
typically make extensive use of Unix-specific calls.  Consequently, I
have absolutely no interest in making my code compatible with Jython
or IronPython, since Jython is stuck at 2.2, IronPython at 2.4, and
neither provide full support for the Python Standard Library or access
to Unix-specific functionality.

I might at some point want to write some Jython code to make use of
Java libraries, but when I code in Jython, I will have absolutely no
interest in trying to make that code compatible with CPython, since
that cannot be if my Jython code calls libraries that are not
available to CPython.

>>> Exactly like you'd be doing if you rely on the value of

>>> uninitialized variables in C.

>> Exactly like I'd be doing if I made Unix system calls in my C code.
>> After all, system calls are implementation dependent, aren't they?
>> That doesn't mean that I don't rely on them every day.

> That depends on whether you program to a specific standard or not.

What standard would that be?  Posix is too restrictive.
BSD/OSX/Linux/Solaris are all different.  I make my program work on
the platform I'm writing it for (keeping in mind what platforms I
might want to port to in the future, in order to avoid obvious
portability pitfalls), and then if the program needs to be ported
eventually to another platforms, I figure out how to do that when the
time comes.

>>> Of course not. Because they *do* guarantee and specify that. C
>>> doesn't, and neither does Python.

>> CPython does by tradition *and* by popular will.

> But you make the mistake of assuming that Python is CPython, which it isn't.

I do not make that mistake.  I refer to CPython as "Python" as does
99% of the Python community.  When I talk about Jython, I call in
"Jython" and when I talk about "IronPython" I refer to it as
"IronPython".  None of this implies that I don't understand that
CPython has features in it that a more strict interpretation of the
word "Python" doesn't necessarily have, just as when I call a tomato a
"vegetable" that doesn't mean that I don't understand that it is
really a fruit.

>> Also the language reference manual specifically indicates that
>> CPython uses a refcounter and documents that it collects objects as
>> soon as they become unreachable (with the appropriate caveats about
>> circular references, tracing, debugging, and stored tracebacks).

> Indeed, but that *is* implementation dependent. As long as you stick
> to CPython you'll be fine. That's allowed. Just be careful about the
> discussions you get into :-)

I've stated over and over again that all I typically care about is
CPython, and what I'm criticized for is for my choice to program for
CPython, rather than for a more generalized least-common-denominator
"Python".

When I program for C++, I also program for the compilers and OS'es
that I will be using, as trying to write C++ code that will compile
under all C++ compilers and OS'es is an utterly losing proposition.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

>> And why would you do that?  People rely very heavily in C++ on when
>> destructors will be called, and they are in fact encouraged to do so.
>> They are, in fact, encouraged to do so *so* much that constructs like
>> "finally" and "with" have been rejected by the C++ BDFL.  Instead, you
>> are told to use smart pointers, or what have you, to clean up your
>> allocated resources.

> For the record, C++ doesn't have a BDFL.

Yes, I know.

   http://dictionary.reference.com/browse/analogy

> And yes, I know that it's used all the time in C++ and is heavily
> encouraged. However, C++ has totally different object semantics than
> Python,

That would depend on how you program in C++.  If you use a framework
based on refcounted smart pointers, then it is rather similar.
Especially if you back that up in your application with a conservative
garbage collector, or what have you.

>> I so no reason not to make Python at least as expressive a programming
>> language as C++.

> I have an overwhelming urge to say something vulgar here. I'm going
> to restrain myself and point out that this isn't a discussion about
> expressiveness.

Says who?

>> That's a poor analogy.  C doesn't guarantee that pointers will be
>> initialized to 0, and in fact, they typically are not.  CPython, on
>> other other hand, guarantees that the refcounter behaves a certain
>> way.

> It's a perfect analogy, because the value of an uninitialized pointer
> in C is *implementation dependent*.

Name one common C implementation that guarantees that uninitialized
pointers will be initialized to null.  None that I have *ever* used
make such a guarantee.  In fact, uninitialized values have always been
garbage with every C compiler I have ever used.

If gcc guaranteed that uninitialized variables would always be zeroed,
and you knew that your code would always be compiled with gcc, then
you would be perfectly justified in coding in a style that assumed
null values for uninitialized variables.  Those are some big if's,
though.

> The Python language reference explicitly does *not* guarantee the
> behavior of the refcounter.

Are you suggesting that it is likely to change?  If so, I think you
will find a huge uproar about it.

> By relying on it, you are relying on an implementation specific,
> non-specified behavior.

I'm relying on a feature that has worked fine since the early '90s,
and if it is ever changed in the future, I'm sure that plenty of other
language changes will come along with it that will make adapting code
that relies on this feature to be the least of my porting worries.

> Exactly like you'd be doing if you rely on the value of
> uninitialized variables in C.

Exactly like I'd be doing if I made Unix system calls in my C code.
After all, system calls are implementation dependent, aren't they?
That doesn't mean that I don't rely on them every day.

>> There are languages other than C that guarantee that values are
>> initialized in certain ways.  Are you going to also assert that in
>> those languages you should not rely on the initialization rules?

> Of course not. Because they *do* guarantee and specify that. C
> doesn't, and neither does Python.

CPython does by tradition *and* by popular will.

Also the language reference manual specifically indicates that CPython
uses a refcounter and documents that it collects objects as soon as
they become unreachable (with the appropriate caveats about circular
references, tracing, debugging, and stored tracebacks).

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

> Sure, but thats part of the general refcounting vs GC argument -
> refcounting gives (a certain level of) timeliness in resource
> collection, GC often only runs under memory pressure. If you're
> saying that we should keep refcounting because it provides better
> handling of non-memory limited resources like file handles, I
> probably wouldn't argue. But saying we should keep refcounting
> because people like to and should write code that relies on implicit
> scope level object destruction I very strongly argue against.

And why would you do that?  People rely very heavily in C++ on when
destructors will be called, and they are in fact encouraged to do so.
They are, in fact, encouraged to do so *so* much that constructs like
"finally" and "with" have been rejected by the C++ BDFL.  Instead, you
are told to use smart pointers, or what have you, to clean up your
allocated resources.

I so no reason not to make Python at least as expressive a programming
language as C++.

>> > Relying on the specific semantics of refcounting to give
>> > certain lifetimes is a logic error.
>> >
>> > For example:
>> >
>> > f = some_file() #maybe it's the file store for a database implementation
>> > f.write('a bunch of stuff')
>> > del f
>> > #insert code that assumes f is closed.

>> That's not a logic error if you are coding in CPython, though I agree
>> that in this particular case the explicit use of "with" would be
>> preferable due to its clarity.

> I stand by my statement. I feel that writing code in this manner is
> like writing C code that assumes uninitialized pointers are 0 -
> regardless of whether it works, it's erroneous and bad practice at
> best, and actively harmful at worst.

That's a poor analogy.  C doesn't guarantee that pointers will be
initialized to 0, and in fact, they typically are not.  CPython, on
other other hand, guarantees that the refcounter behaves a certain
way.

There are languages other than C that guarantee that values are
initialized in certain ways.  Are you going to also assert that in
those languages you should not rely on the initialization rules?

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

>> Some people here have been arguing that all code should use "with" to
>> ensure that the files are closed.  But this still wouldn't solve the
>> problem of the large data structures being left around for an
>> arbitrary amount of time.

> I don't think anyone has suggested that. Let me be clear about *my*
> position: When you need to ensure that a file has been closed by a
> certain time, you need to be explicit about it. When you don't care,
> just that it will be closed "soonish" then relying on normal object
> lifetime calls is sufficient. This is true regardless of whether
> object lifetimes are handled via refcount or via "true" garbage
> collection.

But it's *not* true at all when relying only on a "true GC"!  Your
program could easily run out of file descriptors if you only have a
real garbage collector and code this way (and are opening lots of
files).  This is why destructors are useless in Java -- you can't rely
on them *ever* being called.  In Python, however, destructors are
quite useful due to the refcounter.

> Relying on the specific semantics of refcounting to give
> certain lifetimes is a logic error.
>
> For example:
>
> f = some_file() #maybe it's the file store for a database implementation
> f.write('a bunch of stuff')
> del f
> #insert code that assumes f is closed.

That's not a logic error if you are coding in CPython, though I agree
that in this particular case the explicit use of "with" would be
preferable due to its clarity.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

>> Also, any caught exception is automatically cleared when
>> the catching procedure returns anyway, so it's not like Python has
>> ever considered a caught exception to be precious information that
>> ought to be preserved long past the point where it is handled.

> That's the point. Python takes care of clearing the traceback. Calls
> to exc_clear are rarely seen.

But that's probably because it's very rare to catch an exception and
then not return quickly.  Typically, the only place this would happen
is in main(), or one of its helpers.

> If they are simply a performance tweak then it's not an issue *. I
> was just concerned that the calls were necessary to keep resources
> from being exhausted.

Well, if you catch an exception and don't return quickly, you have to
consider not only the possibility that there could be some open files
left in the traceback, but also that there could be a large and now
useless data structures stored in the traceback.

Some people here have been arguing that all code should use "with" to
ensure that the files are closed.  But this still wouldn't solve the
problem of the large data structures being left around for an
arbitrary amount of time.

> But some things will make it into ISO Python.

Is there a movement afoot of which I'm unaware to make an ISO standard
for Python?

> Just as long as you have weighed the benefits against a future move
> to a JIT-accelerated, continuation supporting PyPy interpreter that
> might not use reference counting.

I'll worry about that day when it happens, since many of my calls to
the standard library will probably break anyway at that point.  Not to
mention that I don't stay within the confines of Python 2.2, which is
where Jython currently is.  (E.g., Jython does not have generators.)
Etc.

>> I think that Jython and IronPython, etc., should adopt these
>> semantics, but I imagine they might not for performance reasons.  I
>> don't generally use Python for it's speediness, however, but rather
>> for it's pleasant syntax and semantics and large, effective
>> library.

> Yet improved performance appeared to be a priority in Python 2.4
> development, and Python's speed continues to be a concern.

I don't think the refcounting semantics should slow Python down much
considering that it never has aimed for C-level performance anyway.
(Some people claim it's a drag on supporting threads.  I'm skeptical,
though.)  I can see it being a drag on something like Jython, though,
were you are going through a number of different layers to get from
Jython code to the hardware.

Also, I imagine that no one wants to put in the work in Jython to have
a refcounter when the garbage collector comes with the JVM for free.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

>> I'm not sure I understand you here.  You're saying that I should have
>> the foresight to wrap all my file opens is a special class to
>> facilitate debugging?

> Obviously you had the foresight to realize with statements could
> compromise debugging. I never considered it myself.

It's not really so much a matter of having foresight, as much as
having had experience debugging a fair amount of code.  And, at times,
having benefited from the traditional idiomatic way of coding in
Python, where files are not explicitly closed.

Since there are benefits with the typical coding style, and I find
there to be no significant downside, other than if, perhaps some code
holds onto tracebacks, I suggest that the problem be idiomatically
addressed in the *few* code locations that hold onto tracebacks,
rather than in all the *myriad* code locations that open and close
files.

>> Or are you suggesting that early in __main__.main(), when I wish to
>> debug something, I do something like:
>>__builtins__.open = __builtins__.file = MyFile
>> ?
>> I suppose that would work.

> No, I would never suggest replacing a builtin like that. Even
> replacing a definite hook like __import__ is risky, should more than
> one package try and do it in a program.

That misinterpretation of your idea would only be reasonable while
actually debugging, not for standard execution.  Standard rules of
coding elegance don't apply while debugging, so I think the
misinterpretation might be a reasonable alternative.  Still I think
I'd just prefer to stick to the status quo in this regard.

> As long as the code isn't dependent on explicitly cleared
> exceptions. But if it is I assume it is well documented.

Typically the resource in question is an open file.  These usually
don't have to be closed in a particularly timely fashion.  If, for
some reason, a files absolutelys need to be closed rapidly, then it's
probably best to use "with" in such a case.  Otherwise, I vote for the
de facto standard idiom of relying on the refcounter along with
explicitly clearing exceptions in the situations we've previously
discusses.

If some code doesn't explicitly clear an exception, though, and holds
onto the the most recent one while running in a loop (or what have
you), in the cases we are considering, it hardly seems like the end of
the world.  It will just take a little bit longer for a single file to
be closed than might ideally be desired.  But this lack of ideal
behavior is usually not going to cause much trouble.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

>> You don't necessarily want a function that raises an exception to
>> deallocate all of its resources before raising the exception, since
>> you may want access to these resources for debugging, or what have
>> you.

> No problem:
>
> [...]
>
>  >>> class MyFile(file):
>   def __exit__(self, exc_type, exc_val, exc_tb):
>   if exc_type is not None:
>   self.my_last_posn = self.tell()
>   return file.__exit__(self, exc_type, exc_val, exc_tb)

I'm not sure I understand you here.  You're saying that I should have
the foresight to wrap all my file opens is a special class to
facilitate debugging?

If so, (1) I don't have that much foresight and don't want to have
to.  (2) I debug code that other people have written, and they often
have less foresight than me.  (3) It would make my code less clear to
ever file open wrapped in some special class.

Or are you suggesting that early in __main__.main(), when I wish to
debug something, I do something like:

   __builtins__.open = __builtins__.file = MyFile

?

I suppose that would work.  I'd still prefer to clear exceptions,
though, in those few cases in which a function has caught an exception
and isn't going to be returning soon and have the resources generally
kept alive in the traceback.  To me, that's the more elegant and
general solution.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

>>> Explicitly clear the exception? With sys.exc_clear?

>> Yes.  Is there a problem with that?

> As long as nothing tries to re-raise the exception I doubt it breaks
> anything:
>
>  >>> import sys
>  >>> try:
>   raise StandardError("Hello")
> except StandardError:
>   sys.exc_clear()
>   raise
>
>
> Traceback (most recent call last):
>File "", line 5, in 
>  raise
> TypeError: exceptions must be classes, instances, or strings
> (deprecated), not NoneType

I guess I don't really see that as a problem.  Exceptions should
normally only be re-raised where they are caught.  If a piece of code
has decided to handle an exception, and considers it dealt with, there
is no reason for it not to clear the exception, and good reason for it
to do so.  Also, any caught exception is automatically cleared when
the catching procedure returns anyway, so it's not like Python has
ever considered a caught exception to be precious information that
ought to be preserved long past the point where it is handled.

> But it is like calling the garbage collector. You are tuning the
> program to ensure some resource isn't exhausted.

I'm not sure I see the analogy: Calling the GC can be expensive,
clearing an exception is not.  The exception is going to be cleared
anyway when the procedure returns, the GC wouldn't likely be.

It's much more like explicitly assigning None to a variable that
contains a large data structure when you no longer need the contents
of the variable.  Doing this sort of thing can be a wise thing to do
in certain situations.

> It relies on implementation specific behavior to be provably
> reliable*.

As Python is not a formally standardized language, and one typically
relies on the fact that CPython itself is ported to just about every
platform known to Man, I don't find this to be a particular worry.

> If this is indeed the most obvious way to do things in your
> particular use case then Python, and many other languages, is
> missing something. If the particular problem is isolated,
> formalized, and general solution found, then a PEP can be
> submitted. If accepted, this would ensure future and cross-platform
> compatibility.

Well, I think that the refcounting semantics of CPython are useful,
and allow one to often write simpler, easier-to-read and maintain
code.  I think that Jython and IronPython, etc., should adopt these
semantics, but I imagine they might not for performance reasons.  I
don't generally use Python for it's speediness, however, but rather
for it's pleasant syntax and semantics and large, effective library.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

I wrote:

> P.S.  The last time I took a language class (about five or six years
> ago), the most interesting languages I thought were descended from
> Self, not any functional language.  (And Self, of course is descended
> from Smalltalk, which is descended from Lisp.)

I think that Cecil is the particular language that I was most thinking
of:

   http://en.wikipedia.org/wiki/Cecil_programming_language

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

> Explicitly clear the exception? With sys.exc_clear?

Yes.  Is there a problem with that?

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin  writes:

> Haskell and ML are both evaluate typed lambda calculus unlike Lisp
> which is based on untyped lambda calculus.  Certainly the most
> familiar features of Lisp (dynamic typing, S-expression syntax,
> programs as data (Lisp's macro system results from this)) are absent
> from Haskell and ML.

And that is supposed to make them better and more flexible???  The
ideal language of the future will have *optional* manifest typing
along with type-inference, and will have some sort of pramgma to turn
on warnings when variables are forced to become dynamic due to there
not being enough type information to infer the type.  But it will
still allow programming with dynamic typing when that is necessary.

The last time I looked at Haskell, it was still in the stage of being
a language that only an academic could love.  Though, it was certainly
interesting.

> Haskell's type system lets it do stuff that Lisp can't approach.

What kind of stuff?  Compile-time polymorphism is cool for efficiency
and type safety, but doesn't actually provide you with any extra
functionality that I'm aware of.

> I'm reserving judgement about whether Haskell is really practical
> for application development, but it can do stuff that no traditional
> Lisp can (e.g. its concurrency and parallelism stuff, with
> correctness enforced by the type system).  It makes it pretty clear
> that Lisp has become Blub.

Where do you get this idea that the Lisp world does not get such
things as parallelism?  StarLisp was designed for the Connection
Machine by Thinking Machines themselves.  The Connection Machine was
one of the most parallel machines ever designed.  Alas, it was ahead of
it's time.

Also, I know a research scientist at CSAIL at MIT who has designed and
implemented a version of Lisp for doing audio and video art.  It was
designed from the ground-up to deal with realtime audio and video
streams as first class objects.  It's actually pretty incredible -- in
just a few lines of code, you can set up a program that displays the
same video multiplied and tiled into a large grid of little videos
tiles, but where a different filter or delay is applied to each tile.
This allows for some stunningly strange and interesting video output.
Similar things can be done in the language with music (though if you
did that particular experiment it would probably just sound
cacophonous).

Does that sound like an understanding of concurrency to you?  Yes, I
thought so.

Also, Dylan has optional manifests types and type inference, so the
Lisp community understands some of the benefits of static typing.
(Even MacLisp had optional manifest types, but they weren't  there for
safety, but rather for performance.  Using them, you could get Fortran
level of performance out of Lisp, which was quite a feat at the time.)

> ML's original implementation language is completely irrelevant;
> after all Python is still implemented in C.

Except that in the case of ML, it was mostly just a thin veneer on
Lisp that added a typing system and type inference.

>> Also, Scheme has a purely functional core (few people stick to it, of
>> course), and there are purely functional dialects of Lisp.

> Scheme has never been purely functional.  It has had mutation since
> the beginning.

I never said that was purely functional -- I said that it has a purely
functional core.  I.e., all the functions that have side effects have
and "!" on their ends (or at least they did when I learned the
language), and there are styles of programming in Scheme that
discourage using any of those functions.

|>oug

P.S.  The last time I took a language class (about five or six years
ago), the most interesting languages I thought were descended from
Self, not any functional language.  (And Self, of course is descended
from Smalltalk, which is descended from Lisp.)
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin <http://[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>> P.S. Besides Haskell is basically a refinement of ML, which is a
>> dialect of Lisp.

> I'd say Haskell and ML are descended from Lisp, just like mammals are
> descended from fish.

Hardly -- they all want to share the elegance of lambda calculus,
n'est-ce pas?  Also, ML was originally implemented in Lisp, and IIRC
correctly, at least in early versions, shared much of Lisp's syntax.

Also, Scheme has a purely functional core (few people stick to it, of
course), and there are purely functional dialects of Lisp.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Michele Simionato <[EMAIL PROTECTED]> writes:

>> Right.  So?  I understand this issue completely and I code
>> accordingly.

> What does it mean you 'code accordingly'? IMO the only clean way out
> of this issue is to NOT rely on the garbage collector and to manage
> resource deallocation explicitely, not implicitely.

(1) I don't rely on the refcounter for resources that ABSOLUTELY,
POSITIVELY must be freed before the scope is left.  In the code that
I've worked on, only a small fraction of resources would fall into
this category.  Open files, for instance, rarely do.  For open files,
in fact, I actually want access to them in the traceback for debugging
purposes, so closing them using "with" would be the opposite of what I
want.

(2) I don't squirrel away references to tracebacks.

(3) If a procedure catches an exception but isn't going to return
quickly, I clear the exception.

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin <http://[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>> But that's a library issue, not a language issue.  The technology
>> exists completely within Lisp to accomplish these things, and most
>> Lisp programmers even know how to do this, as application frameworks
>> in Lisp often do this kind.  The problem is getting anything put into
>> the standard.  Standardizing committees just suck.

> Lisp is just moribund, is all.  Haskell has a standardizing committee
> and yet there are lots of implementations taking the language in new
> and interesting directions all the time.  The most useful extensions
> become de facto standards and then they make it into the real
> standard.

You only say this because you are not aware of all the cool dialetcs
of Lisp that are invented.  The problem is that they rarely leave the
tiny community that uses them, because each community comes up with
it's own different cool dialect of Lisp.  So, clearly the issue is not
one of any lack of motivation or people working on Lisp innovations --
it's getting them to sit down together and agree on a standard.

This, of course is a serious problem.  One that is very similar to the
problem with Python vs. Ruby on Rails.  It's not the problem that you are
ascribing to Lisp, however.

|>oug

P.S. Besides Haskell is basically a refinement of ML, which is a
dialect of Lisp.

P.P.S. I doubt that any day soon any purely (or even mostly)
functional language is going to gain any sort of popularity outside of
academia.  Maybe 20 years from now, they will, but I wouldn't bet on
it.
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Lenard Lindstrom <[EMAIL PROTECTED]> writes:

> Douglas Alan wrote:

>> [I]n Python, you can be 100% sure that your files
>> will be closed in a timely manner without explicitly closing them, as
>> long as you are safe in making certain assumptions about how your code
>> will be used.  Such assumptions are called "preconditions", which are
>> an understood notion in software engineering and by me when I write
>> software.

> So documenting an assumption is more effective than removing the
> assumption using a with statement?

Once again I state that I have nothing against "with" statements.  I
used it all the time ages ago in Lisp.

But (1) try/finally blocks were not to my liking for this sort of
thing because they are verbose and I think error-prone for code
maintenance.  I and many others prefer relying on the refcounter for
file closing over the try/finally solution.  Consequently, using the
refcounter for such things is a well-entrenched and succinct idiom.
"with" statements are a big improvement over try/finally, but for
things like file closing, it's six of one, half dozen of the other
compared against just relying on the refcounter.

(2) "with" statements do not work in all situations because often you
need to have an open file (or what have you) survive the scope in
which it was opened.  You may need to have multiple objects be able to
read and/or write to the file.  And yet, the file may not want to be
kept open for the entire life of the program.  If you have to decide
when to explicitly close the file, then you end up with the same sort
of modularity issues as when you have to free memory explicitly.  The
refcounter handles these sorts of situations with aplomb.

(3) Any code that is saving tracebacks should assume that it is likely
to cause trouble, unless it is using code that is explicitly
documented to be robust in the face of this, just as any code that
wants to share objects between multiple threads should assume that
this is likely to cause trouble, unless it is using code that is
explicitly documented to be robust in the face of this.

(4) Any code that catches exceptions should either return soon or
clear the exception.  If it doesn't, the problem is not with the
callee, but with the caller.

(5) You don't necessarily want a function that raises an exception to
deallocate all of its resources before raising the exception, since
you may want access to these resources for debugging, or what have you.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Duncan Booth <[EMAIL PROTECTED]> writes:

>> A precondition of much of my Python code is that callers won't
>> squirrel away large numbers of tracebacks for long periods of time.  I
>> can live with that.  Another precondition of much of my code is that
>> the caller doesn't assume that it is thread-safe.  Another
>> precondition is that the caller doesn't assume that it is likely to
>> meet real-time constraints.  Another precondition is that the caller
>> doesn't need my functions to promise not to generate any garbage that
>> might call the GC to invoked.

> None of that is relevant.

Of course it is.  I said "large number of tracebacks" up there, and
you promptly ignored that precondition in your subsequent
counterexample.

> Have you ever seen any code looking roughly like this?

> def mainloop():
>while somecondition:
>   try:
>   dosomestuff()
>   except SomeExceptions:
>   handletheexception()

Of course.

> Now, imagine somewhere deep inside dosomestuff an exception is
> raised while you have a file open and the exception is handled in
> mainloop. If the loop then continues with a fresh call to
> dosomestuff the traceback object will continue to exist until the
> next exception is thrown or until mainloop returns.

It's typically okay in my software for a single (or a few) files to
remain open for longer than I might expect.  What it couldn't handle
is running out of file descriptors, or the like.  (Just like it
couldn't handle running out of memory.)  But that's not going to
happen with your counterexample.

If I were worried about a file or two remaining open too long, I'd
clear the exception in the mainloop above, after handling it.  Python
lets you do that, doesn't it?

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steve Holden <[EMAIL PROTECTED]> writes:

> "Python" doesn't *have* any refcounting semantics.

I'm not convinced that Python has *any* semantics at all outside of
specific implementations.  It has never been standardized to the rigor
of your typical barely-readable language standards document.

> If you rely on the behavior of CPython's memory allocation and
> garbage collection you run the risk of producing programs that won't
> port tp Jython, or IronPython, or PyPy, or ...

> This is a trade-off that many users *are* willing to make.

Yes, I have no interest at the moment in trying to make my code
portable between every possible implementation of Python, since I have
no idea what features such implementations may or may not support.
When I code in Python, I'm coding for CPython.  In the future, I may
do some stuff in Jython, but I wouldn't call it "Python" -- it'd call
it "Jython".  When I do code for Jython, I'd be using it to get to
Java libraries that would make my code non-portable to CPython, so
portability here seems to be a red herring.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Hrvoje Niksic <[EMAIL PROTECTED]> writes:

>> Generators aren't slower than hand-coded iterators in *Python*, but
>> that's because Python is a slow language.

> But then it should be slow for both generators and iterators.

Python *is* slow for both generators and iterators.  It's slow for
*everything*, except for cases when you can have most of the work done
within C-coded functions or operations that perform a lot of work
within a single call.  (Or, of course, cases where you are i/o
limited, or whatever.)

>> *Perhaps* there would be some opportunities for more optimization if
>> they had used a less general mechanism.)

> Or if the generators were built into the language and directly
> supported by the compiler.  In some cases implementing a feature is
> *not* a simple case of writing a macro, even in Lisp.  Generators may
> well be one such case.

You can't implement generators in Lisp (with or without macros)
without support for generators within the Lisp implementation.  This
support was provided as "stack groups" on Lisp Machines and as
continuations in Scheme.  Both stack groups and continuations are
slow.  I strongly suspect that if they had provided direct support for
generators, rather than indirectly via stack groups and continuations,
that that support would have been slow as well.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Jean-Paul Calderone <[EMAIL PROTECTED]> writes:

>>On the other hand, in Python, you can be 100% sure that your files
>>will be closed in a timely manner without explicitly closing them, as
>>long as you are safe in making certain assumptions about how your code
>>will be used.  Such assumptions are called "preconditions", which are
>>an understood notion in software engineering and by me when I write
>>software.

> You realize that Python has exceptions, right?

Yes, of course.

> Have you ever encountered a traceback object?

Yes, of course.

> Is one of your preconditions that no one will ever handle an
> exception raised by your code or by their own code when it is
> invoked by yours?

A precondition of much of my Python code is that callers won't
squirrel away large numbers of tracebacks for long periods of time.  I
can live with that.  Another precondition of much of my code is that
the caller doesn't assume that it is thread-safe.  Another
precondition is that the caller doesn't assume that it is likely to
meet real-time constraints.  Another precondition is that the caller
doesn't need my functions to promise not to generate any garbage that
might call the GC to invoked.

If I had to write all my code to work well without making *any*
assumptions about what the needs of the caller might be, then my code
would have to be much more complicated, and then I'd spend more effort
making my code handle situations that it won't face for my purposes.
Consequently, I'd have less time to make my software have the
functionality that I actually require.

Regarding, specifically, tracebacks holding onto references to open
files -- have you considered that you may actually *want* to see the
file in the state that it was in when the exception was raised for the
purposes of debugging, rather than having it forcefully closed on you?

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

>> On the other hand, in Python, you can be 100% sure that your files
>> will be closed in a timely manner without explicitly closing them, as
>> long as you are safe in making certain assumptions about how your code
>> will be used.  Such assumptions are called "preconditions", which are
>> an understood notion in software engineering and by me when I write
>> software.

> Next time theres one of those "software development isn't really
> engineering" debates going on I'm sure that we'll be able to settle
> the argument by pointing out that relying on *explicitly* unreliable
> implementation details is defined as "engineering" by some people.

The proof of the pudding is in it's eating.  I've worked on very large
programs that exhibited very few bugs, and ran flawlessly for many
years.  One managed the memory remotely of a space telescope, and the
code was pretty tricky.  I was sure when writing the code that there
would be a number of obscure bugs that I would end up having to pull
my hair out debugging, but it's been running flawlessly for more than
a decade now, without require nearly any debugging at all.

Engineering to a large degree is knowing where to dedicate your
efforts.  If you dedicate them to where they are not needed, then you
have less time to dedicate them to where they truly are.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

> You're arguing against explicit resource management with the argument
> that you don't need to manage resources. Can you not see how
> ridiculously circular this is?

No.  It is insane to leave files unclosed in Java (unless you know for
sure that your program is not going to be opening many files) because
you don't even know that the garbage collector will ever even run, and
you could easily run out of file descriptors, and hog system
resources.

On the other hand, in Python, you can be 100% sure that your files
will be closed in a timely manner without explicitly closing them, as
long as you are safe in making certain assumptions about how your code
will be used.  Such assumptions are called "preconditions", which are
an understood notion in software engineering and by me when I write
software.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Dennis Lee Bieber <[EMAIL PROTECTED]> writes:

>   LISP and FORTH are cousins...

Not really.  Their only real similarity (other than the similarities
shared by most programming languages) is that they both use a form of
Polish notation.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Michele Simionato <[EMAIL PROTECTED]> writes:

>> I've written plenty of Python code that relied on destructors to
>> deallocate resources, and the code always worked.

> You have been lucky:

No I haven't been lucky -- I just know what I'm doing.

> $ cat deallocating.py
> import logging
>
> class C(object):
> def __init__(self):
> logging.warn('Allocating resource ...')
>
> def __del__(self):
> logging.warn('De-allocating resource ...')
> print 'THIS IS NEVER REACHED!'
>
> if __name__ == '__main__':
> c = C()
>
> $ python deallocating.py
> WARNING:root:Allocating resource ...
> Exception exceptions.AttributeError: "'NoneType' object has no
> attribute 'warn'" in  0xb7b9436c>> ignored

Right.  So?  I understand this issue completely and I code
accordingly.

> Just because your experience has been positive, you should not
> dismiss the opinion who have clearly more experience than you on
> the subtilities of Python.

I don't dismiss their opinion at all.  All I've stated is that for my
purposes I find that the refcounting semantics of Python to be useful,
expressive, and dependable, and that I wouldn't like it one bit if
they were removed from Python.

Those who claim that the refcounting semantics are not useful are the
ones who are dismissing my experience.  (And the experience of
zillions of other Python programmers who have happily been relying on
them.)

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Hrvoje Niksic <[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>> I think you overstate your case.  Lispers understand iteration
>> interfaces perfectly well, but tend to prefer mapping fuctions to
>> iteration because mapping functions are both easier to code (they
>> are basically equivalent to coding generators) and efficient (like
>> non-generator-implemented iterators).  The downside is that they are
>> not quite as flexible as iterators (which can be hard to code) and
>> generators, which are slow.

> Why do you think generators are any slower than hand-coded iterators?

Generators aren't slower than hand-coded iterators in *Python*, but
that's because Python is a slow language.  In a fast language, such as
a Lisp, generators are like 100 times slower than mapping functions.
(At least they were on Lisp Machines, where generators were
implemented using a more generator coroutining mechanism [i.e., stack
groups].  *Perhaps* there would be some opportunities for more
optimization if they had used a less general mechanism.)

CLU, which I believe is the language that invented generators, limited
them to the power of mapping functions (i.e., you couldn't have
multiple generators instantiated in parallel), making them really
syntactic sugar for mapping functions.  The reason for this limitation
was performance.  CLU was a fast language.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

> Obviously. But theres nothing about the with statement that's
> different than using smart pointers in this regard.

Sure there is -- smart pointers handle many sorts of situations, while
"with" only handles the case where the lifetime of the object
corresponds to the scope.

>> But I don't typically wish for this sort of intent to be made
>> explicit.  TMI!  I used "with" for *many* years in Lisp, since this
>> is how non-memory resource deallocation has been dealt with in Lisp
>> since the dawn of time.  I can tell you from many years of
>> experience that relying on Python's refcounter is superior.

> I question the relevance of your experience, then.

Gee, thanks.

> Refcounting is fine for memory, but as you mention below, memory is
> only one kind of resource and refcounting is not necessarily the
> best technique for all resources.

I never said that it is the best technique for *all* resources.  Just
the most typical ones.

>> This just isn't true.  For many years I have not had to explicitly
>> close files in Python.  Nor have I had to do so in C++.  They have
>> been closed for me implicitly.  "With" is not implicit -- or at least
>> not nearly as implicit as was previous practice in Python, or as is
>> current practice in C++.

> You still don't have to manually close files. But you cannot, and
> never could, rely on them being closed at a given time unless you
> did so.

You could for most intents and purposes.

> If you need a file to be closed in a deterministic manner, then you
> must close it explicitly.

You don't typically need them to be closed in a completely fool-proof
deterministic fashion.  If some other code catches your exceptions and
holds onto the traceback then it must know that can be delaying a few
file-closings, or the like.

> The with statement is not implicit and never has been. Implicit
> resource management is *insufficient* for the general resource
> management case. It works fine for memory, it's okay for files
> (until it isn't), it's terrible for thread locks and network
> connections and database transactions. Those things require
> *explicit* resource management.

Yes, I agree there are certain situations in which you certainly want
"with", or something like it.  I've never disagreed with that
assertion at all.  I just don't agree that for most Python code this
is the *typical* case.

> To the extent that your code ever worked when you relied on this
> detail, it will continue to work.

I've written plenty of Python code that relied on destructors to
deallocate resources, and the code always worked.

> There are no plans to replace pythons refcounting with fancier GC
> schemes that I am aware of.

This is counter to what other people have been saying.  They have been
worrying me by saying that the refcounter may go away and so you may
not be able to rely on predictable object lifetimes in the future.

> Nothing about Pythons memory management has changed. I know I'm
> repeating myself here, but you just don't seem to grasp this
> concept.  Python has *never* had deterministic destruction of
> objects. It was never guaranteed, and code that seemed like it
> benefited from it was fragile.

It was not fragile in my experience.  If a resource *positively*,
*absolutely* needed to be deallocated at a certain point in the code
(and occasionally that was the case), then I would code that way.  But
that has been far from the typical case for me.

>> Purify tells me that I know more about the behavior of my code than
>> you do: I've *never* had any memory leaks in large C++ programs that
>> used refcounted smart pointers that were caused by cycles in my data
>> structures that I didn't know about.

> I'm talking about Python refcounts. For example, a subtle resource
> leak that has caught me before is that tracebacks hold references to
> locals in the unwound stack.

Yes, I'm aware of that.  Most programs don't hold onto tracebacks for
long.  If you are working with software that does, then, I agree, that
sometimes one will have to code things more precisely.

> If you relied on refcounting to clean up a resource, and you needed
> exception handling, the resource wasn't released until *after* the
> exception unwound, which could be a problem. Also holding onto
> tracebacks for latter processing (not uncommon in event based
> programs) would artificially extend the lifetime of the resource. If
> the resource you were managing was a thread lock this could be a
> real problem.

Right -- I've always explicitly managed thread locks.

>> I really have no desire to code in C, thank you.  I'd rather be coding
>> in Python.  (Hence my [idle] desire for macros in Python, so that I
>> could do even more of my work in Python.)

> In this particular conversation, I really don't think that theres much
> to say beyond put up or shut up.

I think your attitude here is unPythonic.

> The experts in the field have said that it's not practical.

Gu

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steve Holden <[EMAIL PROTECTED]> writes:

>> Actually, it's "chacun".  And the "à" may precede the "chacun".

>> |>oug

> "chacun" is an elision of the two words "Chaque" (each) and "un"
> (one), and use of those two words is at least equally correct, though
> where it stands in modern usage I must confess I have no idea.

Google can answer that: 158,000 hits for "chaqu'un", 57 million for
"chacun".

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steve Holden <[EMAIL PROTECTED]> writes:

> Douglas Woodrow wrote:

>> On Wed, 27 Jun 2007 01:45:44, Douglas Alan <[EMAIL PROTECTED]> wrote

>>> A chaque son gout

>> I apologise for this irrelevant interruption to the conversation,
>> but this isn't the first time you've written that.  The word
>> "chaque" is not a pronoun.

>> http://grammaire.reverso.net/index_alpha/Fiches/Fiche220.htm

> Right, he probably means "Chaqu'un à son gout" (roughly, each to his
> own taste).

Actually, it's "chacun".  And the "à" may precede the "chacun".

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Dennis Lee Bieber <[EMAIL PROTECTED]> writes:

>   But if these "macros" are supposed to allow one to sort of extend
> Python syntax, are you really going to code things like
>
>   macrolib1.keyword
> everywhere?

No -- I would expect that macros (if done the way that I would like
them to be done) would work something like so:

   from setMacro import macro set, macro let
   let x = 1
   set x += 1

The macros "let" and "set" (like all macro invocations) would have to
be the first tokens on a line.  They would be passed either the
strings "x = 1" and "x += 1", or some tokenized version thereof.
There would be parsing libraries to help them from there.

For macros that need to continue over more than one line, e.g.,
perhaps something like

   let x = 1
   y = 2
   z = 3
   set x = y + z
   y = x + z
   z = x + y
   print x, y, z

the macro would parse up to when the indentation returns to the previous
level.

For macros that need to return values, a new bracketing syntax would
be needed.  Perhaps something like:

   while $(let x = foo()):
  print x

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Douglas Woodrow <[EMAIL PROTECTED]> writes:

> On Wed, 27 Jun 2007 01:45:44, Douglas Alan <[EMAIL PROTECTED]> wrote

>>A chaque son gout

> I apologise for this irrelevant interruption to the conversation, but
> this isn't the first time you've written that.

> The word "chaque" is not a pronoun.

A chacun son epellation.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

> On 6/27/07, Douglas Alan <[EMAIL PROTECTED]> wrote:

>> The C++ folks feel so strongly about this, that they refuse to provide
>> "finally", and insist instead that you use destructors and RAII to do
>> resource deallocation.  Personally, I think that's taking things a bit
>> too far, but I'd rather it be that way than lose the usefulness of
>> destructors and have to use "when" or "finally" to explicitly
>> deallocate resources.

> This totally misrepresents the case. The with statement and the
> context manager is a superset of the RAII functionality.

No, it isn't.  C++ allows you to define smart pointers (one of many
RAII techniques), which can use refcounting or other tracking
techniques.  Refcounting smart pointers are part of Boost and have
made it into TR1, which means they're on track to be included in the
next standard library.  One need not have waited for Boost, as they can
be implemented in about a page of code.

The standard library also has auto_ptr, which is a different sort of
smart pointer, which allows for somewhat fancier RAII than
scope-based.

> It doesn't overload object lifetimes, rather it makes the intent
> (code execution upon entrance and exit of a block) explicit.

But I don't typically wish for this sort of intent to be made
explicit.  TMI!  I used "with" for *many* years in Lisp, since this is
how non-memory resource deallocation has been dealt with in Lisp since
the dawn of time.  I can tell you from many years of experience that
relying on Python's refcounter is superior.

Shouldn't you be happy that there's something I like more about Python
than Lisp?

> Nobody in their right mind has ever tried to get rid of explicit
> resource management - explicit resource management is exactly what you
> do every time you create an object, or you use RAII, or you open a
> file.

This just isn't true.  For many years I have not had to explicitly
close files in Python.  Nor have I had to do so in C++.  They have
been closed for me implicitly.  "With" is not implicit -- or at least
not nearly as implicit as was previous practice in Python, or as is
current practice in C++.

> *Manual* memory management, where the tracking of references and
> scopes is placed upon the programmer, is what people are trying to
> get rid of and the with statement contributes to that goal, it
> doesn't detract from it.

As far as I am concerned, memory is just one resource amongst many,
and the programmer's life should be made easier in dealing with all
such resources.

> Before the with statement, you could do the same thing but you
> needed nested try/finally blocks

No, you didn't -- you could just encapsulate the resource acquisition
into an object and allow the destructor to deallocate the resource.

> RAII is a good technique, but don't get caught up on the
> implementation details.

I'm not -- I'm caught up in the loss of power and elegance that will
be caused by deprecating the use of destructors for resource
deallocation.

> The with statement does exactly the same thing, but is actually
> superior because
>
> a) It doesn't tie the resource managment to object creation. This
> means you can use, for example, with lock: instead of the C++ style
> Locker(lock)

I know all about "with".  As I mentioned above, Lisp has had it since
the dawn of time.  And I have nothing against it, since it is at times
quite useful.  I'm just dismayed at the idea of deprecating reliance
on destructors in favor of "with" for the majority of cases when the
destructor usage works well and is more elegant.

> b) You can tell whether you exited with an exception, and what that
> exception is, so you can take different actions based on error
> conditions vs expected exit. This is a significant benefit, it
> allows the application of context managers to cases where RAII is
> weak. For example, controlling transactions.

Yes, for the case where you might want to do fancy handling of
exceptions raised during resource deallocation, then "when" is
superior, which is why it is good to have in addition to the
traditional Python mechanism, not as a replacement for it.

>> Right, but that doesn't mean that 99.9% of the time, the programmer
>> can't immediately tell that cycles aren't going to be an issue.

> They can occur in the most bizarre and unexpected places. To the point
> where I suspect that the reality is simply that you never noticed your
> cycles, not that they didn't exist.

Purify tells me that I know more about the behavior of my code than
you do: I've *never* had any memory leaks in large C++ programs that
used refco

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Chris Mellon" <[EMAIL PROTECTED]> writes:

> Is this where I get to call Lispers Blub programmers, because they
> can't see the clear benefit to a generic iteration interface?

I think you overstate your case.  Lispers understand iteration
interfaces perfectly well, but tend to prefer mapping fuctions to
iteration because mapping functions are both easier to code (they are
basically equivalent to coding generators) and efficient (like
non-generator-implemented iterators).  The downside is that they are
not quite as flexible as iterators (which can be hard to code) and
generators, which are slow.

Lispers have long since understood how to write mapping function to
iterator converters using stack groups or continuations, but Common
Lisp never mandated stack groups or continuations for conforming
implementations.  Scheme, of course, has continuations, and there are
implementations of Common Lisp with stack groups.

>> The difference is that lisp users can easily define python-like for
>> while python folks have to wait for the implementation.

> Yes, but Python already has it (so the wait time is 0), and the Lisp
> user doesn't.

So do Lispers, provided that they use an implementation of Lisp that
has the aforementioned extensions to the standard.  If they don't,
they are the unfortunately prisoners of the standardizing committees.

And, I guarantee you, that if Python were specified by a standardizing
committee, it would suffer this very same fate.

Regarding there being way too many good but incompatible
implementations of Lisp -- I understand.  The very same thing has
caused Ruby to incredibly rapidly close the lead that Python has
traditionally had over Ruby.  There reason for this is that there are
too many good but incompatible Python web dev frameworks, and only one
good one for Ruby.  So, we see that while Lisp suffers from too much
of a good thing, so does Python, and that may be the death of it if
Ruby on Rails keeps barreling down on Python like a runaway train.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin <http://[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>> > In the Maclisp era functions like mapcar worked on lists, and
>> > generated equally long lists in memory.

>> I'm aware, but there were various different mapping functions.  "map",
>> as opposed to "mapcar" didn't return any values at all, and so you had
>> to rely on side effects with it.

> The thing is there was no standard way in Maclisp to write something
> like Python's "count" function and map over it.  This could be done in
> Scheme with streams, of course.

I'm not sure that you can blame MacLisp for not being object-oriented.
The idea hadn't even been invented yet when MacLisp was implemented
(unless you count Simula).  If someone went to make an OO version of
MacLisp, I'm sure they'd get all this more or less right, and people
have certainly implemented dialects of Lisp that are consistently OO.

>> Right -- I wrote "iterators", not "generators".

> Python iterators (the __iter__ methods on classes) are written with
> yield statements as often as not.

I certainly agree that iterators can be implemented with generators,
but generators are a language feature that are impossible to provide
without deep language support, while iterators are just an OO
interface that any OO language can provide.  Though without a good
macro facility the syntax to use them may not be so nice.

>> That's not ugly.  The fact that CPython has a reference-counting GC
>> makes the lifetime of object predictable, which means that like in
>> C++, and unlike in Java, you can use destructors to good effect.  This
>> is one of the huge boons of C++.  The predictability of lifespan makes
>> the language more expressive and powerful.  The move to deprecate
>> relying on this feature in Python is a bad thing, if you ask me, and
>> removes one of the advantages that Python had over Lisp.

> No that's wrong, C++ has no GC at all, reference counting or
> otherwise, so its destructors only run when the object is manually
> released or goes out of scope.

Right, but implementing generic reference-counted smart pointers is
about a page of code in C++, and nearly every large C++ application
I've seen uses such things.

> Python (as of 2.5) does that using the new "with" statement, which
> finally makes it possible to escape from that losing GC-dependent
> idiom.  The "with" statement handles most cases that C++ destructors
> normally handle.

Gee, that's back to the future with 1975 Lisp technology.  Destructors
are a much better model for dealing with such things (see not *all*
good ideas come from Lisp -- a few come from C++) and I am dismayed
that Python is deprecating their use in favor of explicit resource
management.  Explicit resource management means needlessly verbose
code and more opportunity for resource leaks.

The C++ folks feel so strongly about this, that they refuse to provide
"finally", and insist instead that you use destructors and RAII to do
resource deallocation.  Personally, I think that's taking things a bit
too far, but I'd rather it be that way than lose the usefulness of
destructors and have to use "when" or "finally" to explicitly
deallocate resources.

> Python object lifetimes are in fact NOT predictable because the ref
> counting doesn't (and can't) pick up cyclic structure.

Right, but that doesn't mean that 99.9% of the time, the programmer
can't immediately tell that cycles aren't going to be an issue.

I love having a *real* garbage collector, but I've also dealt with C++
programs that are 100,000+ lines long and I wrote plenty of Python
code before it had a real garbage collector, and I never had any
problem with cyclic data structures causing leaks.  Cycles are really
not all that common, and when they do occur, it's usually not very
difficult to figure out where to add a few lines to a destructor to
break the cycle.

> And the refcounts are a performance pig in multithreaded code,
> because of how often they have to be incremented and updated.

I'm willing to pay the performance penalty to have the advantage of
not having to use constructs like "when".

Also, I'm not convinced that it has to be a huge performance hit.
Some Lisp implementations had a 1,2,3, many (or something like that)
reference-counter for reclaiming short-lived objects.  This bypassed
the real GC and was considered a performance optimization.  (It was
probably on a Lisp Machine, though, where they had special hardware to
help.)

> That's why CPython has the notorious GIL (a giant lock around the
> whole interpreter that stops more than one interpreter thread from

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin  writes:

> Andy Freeman <[EMAIL PROTECTED]> writes:

>> Compare that with what a programmer using Python 2.4 has to do if
>> she'd like the functionality provided by 2.5's with statement.  Yes,
>> with is "just syntax", but it's extremely useful syntax, syntax that
>> can be easily implemented with lisp-style macros.

> Not really.  The with statement's binding targets all have to support
> the protocol, which means a lot of different libraries need redesign.
> You can't do that with macros.

But that's a library issue, not a language issue.  The technology
exists completely within Lisp to accomplish these things, and most
Lisp programmers even know how to do this, as application frameworks
in Lisp often do this kind.  The problem is getting anything put into
the standard.  Standardizing committees just suck.

I just saw a presentation today on the Boost library for C++.  This
project started because the standard library for C++ is woefully
inadequate for today's programming needs, but any chance of getting
big additions into the standard library will take 5-10 years.
Apparently this is true for all computer language standards.  And even
then, the new standard will be seriously lacking, because it is
usually based on armchair thinking rather than real-world usage.

So the Boost guys are making a defacto standard (or so they hope)
library for C++ that has more of the stuff you want, and then when the
standardizing committees get around to revising the actual standard,
the new standard will already be in wide use, meaning they just have
to sign off on it (and perhaps suggest a few tweaks).

Alas, the Lisp standards are stuck in this sort of morass, even while
many implementations do all the right things.

Python doesn't have this problem because it operates like Boost to
begin with, rather than having a zillion implementations tracking some
slow moving standard that then mandates things that might be nearly
impossible to implement, while leaving out much of what people need.

But then again, neither do many dialects of Lisp, which are developed
more or less like Python is.  But then they aren't standards
compliant, and so they don't receive wide adoption.

> Macros can handle some narrow special cases such as file-like
> objects, handled in Python with contextlib.closing.

Macros handle the language part of things in Lisp perfectly well in
this regard.  But you are right -- they certainly can't make
standardizing committees do the right thing.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin <http://[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>> I will certainly admit that Lisp programmers at the time were (and
>> likely still are) much more enamored of mapping functions than of
>> iterators.  Mapping functions certainly get the job done as elegantly
>> as iterators most of the time, although I would agree that they are
>> not quite so general.

> In the Maclisp era functions like mapcar worked on lists, and
> generated equally long lists in memory.

I'm aware, but there were various different mapping functions.  "map",
as opposed to "mapcar" didn't return any values at all, and so you had
to rely on side effects with it.

> It was sort of before my time but I have the impression that Maclisp
> was completely dynamically scoped and as such,

Yes, that's right.

> it couldn't cleanly make anything like generators (since it had no
> way to make lexical closures).

That's right, generators would have been quite difficult to do in
MacLisp.  But a Lisp Machine (with stack groups) could have done them,
and did, with or without closures.

>> Scheme, on, the other hand, at least by idiom, has computation
>> "streams", and streams are equivalent to iterators.

> No not really, they (in SICP) are at best more like class instances
> with a method that mutates some state.  There's nothing like a yield
> statement in the idiom.

Right -- I wrote "iterators", not "generators".

> You could do it with call/cc but SICP just uses ordinary closures to
> implement streams.

Yes, that's right.

>> The canonical way to do this in Lisp would be something like:
>>(mapcar (lambda (x) (blah x)) xs)

> At least you could spare our eyesight by writing that as 
> (mapcar #'blah xs) ;-).

Good point!  But I just love lambda -- even when I'm just using it as
a NOP  (Also I couldn't remember the syntax for accessing the
function property of a symbol in MacLisp.)

> The point is that mapcar (as the name implies) advances down a list
> using cdr, i.e. it only operates on lists, not general iterators or
> streams or whatever.

Right, but each sequence type had it's own corresponding mapping
fuctions.

>> > x = xs[key]

>> I'm not sure what you are asserting?  That Common Lisp doesn't have
>> hash tables?  That's certainly not the case.  Or that it doesn't
>> provide standard generic functions for accessing them

> The latter.  Of course there are getf/setf, but those are necessarily
> macros.

Right.  OO on primitive data types is kind of hard in a non OO
language.  So, when writing an application in MacLisp, or Lisp Machine
lisp, I might have had to spend a bit of time writing an application
framework that provided the OO features I needed.  This was not
particularly hard to do in Lisp, but surely not nearly as nice as if
they had standardized such things.  This would not be particularly
difficult to do, other than the getting everyone to agree on just what
the interfaces should be.  But Lisp programmers, are of course, just
as recalcitrant as Python programmers.

>> A language like Python, which is defined by its implementation,
>> rather than by a standard, can move much more quickly.  This debate
>> though is really one more of what is the best model for language
>> definition, rather than one on what the ideal language is like.

> Python is not Perl and it has in principle always been defined by its
> reference manual,

And in Python's case, the reference manual is just an incomplete
description of the features offered by the implementation, and people
revel in features that are not yet in the reference manual.

> though until fairly recently it's fostered a style of relying on
> various ugly CPython artifacts like the reference counting GC.

That's not ugly.  The fact that CPython has a reference-counting GC
makes the lifetime of object predictable, which means that like in
C++, and unlike in Java, you can use destructors to good effect.  This
is one of the huge boons of C++.  The predictability of lifespan makes
the language more expressive and powerful.  The move to deprecate
relying on this feature in Python is a bad thing, if you ask me, and
removes one of the advantages that Python had over Lisp.

> Lisp accumulated a lot of cruft over the decades and it kept some
> baggage that it really could have done without.

Indeed -- true of most languages.  Of course, there have been quite a
few Lisp dialects that have been cleaned up in quite a few ways (e.g.,
Dylan), but they, of course, have a hard time achieving any
significant traction.

> I don't think Python's designers learned nearly as much from Lisp as
> they could hav

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Alexander Schmolck <[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:

>>> Python has built-in abstractions for a few container types like
>>> lists and dicts, and now a new and more general one (iterators), so
>>> it's the next level up.

>> Common Lisp has had all these things for ages.

> Rubbish. Do you actually know any common lisp?

Yes, though it's been quite a while, and it was mostly on Lisp
Machines, which, at the time, Common Lisp was still being
standardized, and so Lisp Machine "Chine Nual" Lisp wasn't quite
Common Lisp compliant at the time.  Also, Lisp Machine Lisp had a lot
of features, such as stack groups, that weren't put into Common Lisp.
Also, my experience predates CLOS, as at the time Lisp Machines used
Flavors.

Most of my Lisp experience is actually in MacLisp (and Ulisp and
Proto, neither of which you've likely heard of).  MacLisp was an
immediate precursor of Common Lisp, and didn't have a standard object
system at all (I rolled one myself for my applications), but it had
the Loop macro and if I recall correctly, the MacLisp Loop macro
(which was nearly identical to the Chine Nual Loop macro, which I
thought was ported rather unsullied for Common Lisp).  In any case,
IIRC, there were hooks in the Loop macro for dealing with iterators
and I actually used this for providing an iterator-like interface to
generators (for Lisp Machines) that I coded up with macros and stack
groups.

It may be that these hooks didn't make it into the Common Lisp Loop
macro, or that my memory of what was provided by the macro is a little
off.  What's not off, is that it was really easy to implement these
things, and it wasn't like I was some sort of Lisp guru -- I was just
an undergraduate student.

I will certainly admit that Lisp programmers at the time were (and
likely still are) much more enamored of mapping functions than of
iterators.  Mapping functions certainly get the job done as elegantly
as iterators most of the time, although I would agree that they are
not quite so general.  Of course, using generators, I was easily able
to make a converter that would take a mapping function and return a
corresponding iterator.

Scheme, on, the other hand, at least by idiom, has computation
"streams", and streams are equivalent to iterators.

> There is precisely no way to express
>
> for x in xs:
> blah(x)

The canonical way to do this in Lisp would be something like:

   (mapcar (lambda (x) (blah x))
   xs)

Though there would (at least in MacLisp) be a differently named
mapping function for each sequence type, which makes things a bit less
convenient, as you have to know the name of the mapping function
for each type.

> or
> x = xs[key]

I'm not sure what you are asserting?  That Common Lisp doesn't have
hash tables?  That's certainly not the case.  Or that it doesn't
provide standard generic functions for accessing them, so you can
provide your own dictionaries that are implemented differently and
then use exactly the same interface?  The latter I would believe, as
that would be one of my criticisms of Lisp -- although it's pretty
cool that you can load whatever object system you would like (CLOS
being by far the most common), it also means that the core language
itself is a bit deficient in OO terms.

This problem would be significantly mitigated by defining new
standards for such things in terms of CLOS, but unfortunately
standards change unbearably slowly.  There are certainly many
implementations of Lisp that solve these issues, but they have a hard
time achieving wide adoption.  A language like Python, which is
defined by its implementation, rather than by a standard, can move
much more quickly.  This debate though is really one more of
what is the best model for language definition, rather than one on
what the ideal language is like.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Paul Rubin <http://[EMAIL PROTECTED]> writes:

> Douglas Alan <[EMAIL PROTECTED]> writes:
>> And likewise, good macro programming can solve some problems that no
>> amount of linting could ever solve.

> I think Lisp is more needful of macros than other languages, because
> its underlying primitives are too, well, primitive.  You have to write
> all the abstractions yourself.

Well, not really beause you typically use Common Lisp with CLOS and a
class library.  If you ask me, the more things that can (elegantly) be
moved out of the core language and into a standard library, the
better.

> Python has built-in abstractions for a few container types like
> lists and dicts, and now a new and more general one (iterators), so
> it's the next level up.

Common Lisp has had all these things for ages.

> And a bunch of stuff that Python could use macros for, are easily
> done in Haskell using delayed evaluation and monads.  And Haskell is
> starting to grow its own macro system (templates) but that's
> probably a sign that an even higher level language (maybe with
> dependent types or something) would make the templates unnecessary.

Alas, I can't comment too much on Haskell, as, although I am familiar
with it to some extent, I am far from proficient in it.  Don't worry
-- it's on my to-do list.

I think that first I'd like to take Gerry Sussman's new graduate
class, first, though, and I'll find out how it can all be done in
Scheme.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Michele Simionato <[EMAIL PROTECTED]> writes:

> You should really be using pychecker (as well as Emacs autocompletion
> feature ...):

I *do* use Emacs's autocompletion, but sometimes these sorts of bugs
creep in anyway.  (E.g., sometimes I autocomplete in the wrong variable!)

> ~$ pychecker -v x.py
> Processing x...
>
> Warnings...
>
> x.py:4: Variable (longVarableName) not used
>
> [I know you will not be satisfied with this, but pychecker is really
> useful,

Okay, I'll check out PyChecker and PyLint, though I'm sure they will
annoy the hell out of me.  They're probably less annoying than
spending all day tracking down some stupid bug.

> since it catches many other errors that no amount of
> macroprogramming would evere remove].

And likewise, good macro programming can solve some problems that no
amount of linting could ever solve.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Graham Breed <[EMAIL PROTECTED]> writes:

> Another way is to decorate functions with their local variables:

 from strict import my
 @my("item")
> ... def f(x=1, y=2.5, z=[1,2,4]):
> ... x = float(x)
> ... w = float(y)
> ... return [item+x-y for item in z]

Well, I suppose that's a bit better than the previous suggestion, but
(1) it breaks the style rule of not declaring variables until you need
them, and (2) it doesn't catch double initialization.

> The best way to catch false rebindings is to stick a comment with
> the word "rebound" after every statement where you think you're
> rebinding a variable.

No, the best way to catch false rebindings is to have the computers
catch such errors for you.  That's what you pay them for.

> Then you can search your code for cases where there's a "rebound"
> comment but no rebinding.

And how do I easily do that?  And how do I know if I even need to in
the face of sometimes subtle bugs?

> Assuming you're the kind of person who knows that false rebindings
> can lead to perplexing bugs, but doesn't check apparent rebindings
> in a paranoid way every time a perplexing bug comes up, anyway.
> (They aren't that common in modern python code, after all.)

They're not that uncommon, either.

I've certainly had it happen to me on several occasions, and sometimes
they've been hard to find as I might not even see the mispeling even
if I read the code 20 times.

(Like the time I spent all day trying to figure out why my assembly
code wasn't working when I was a student and finally I decided to ask
the TA for help, and while talking him through my code so that he
could tell me what I was doing wrong, I finally noticed the "rO" where
there was supposed to be an "r0".  It's amazing how useful a TA can
be, while doing nothing at all!)

> And you're also the kind of person who's troubled by perplexing bugs
> but doesn't run a fully fledged lint.

Maybe PyLint is better than Lint for C was (hated it!), but my idea of
RAD does not include wading through piles of useless warning messages
looking for the needle warning in the warning haystack.  Or running
any other programs in the midst of my code, run, code, run, ..., loop.

> Maybe that's the kind of person who wouldn't put up with anything
> short of a macro as in the original proposal.  All I know is that
> it's the kind of person I don't want to second guess.

As it is, I code in Python the way that a normal Python programmer
would, and when I have a bug, I track it down through sometimes
painstaking debugging as a normal Python programmer would.  Just as
any other normal Python programmer, I would not use the alternatives
suggested so far, as I'd find them cumbersome and inelegant.  I'd
prefer not to have been bit by the bugs to begin with.  Consequently,
I'd use let and set statements, if they were provided (or if I could
implement them), just as I have the equivalents to let and set in
every other programming language that I commonly program in other than
Python.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

>> You seem oblivious to the fact that one of the huge benefits of Python
>> is its elegant and readable syntax.  The problem with not having a
>> "flexible syntax", is that a programming language can't provide
>> off-the-shelf an elegant syntax for all functionality that will ever
>> be needed.

> It is hardly "off-the-shelf" if somebody has to create new syntax
> for it.

Ummm. that's my point.  No language can provide all the syntax that
will ever be needed to write elegant code.  If module authors can
provide the syntax needed to use their module elegantly, then problem
solved.

>> Eventually programmers find themselves in need of new
>> elegant functionality, but without a corresponding elegant syntax to
>> go along with the new functionality, the result is code that does not
>> look elegant and is therefore difficult to read and thus maintain.

> That's true, as far as it goes, but I think you over-state your
> case.

I do not.

It is so easy for you, without *any* experience with a language (i.e.,
Lisp) or its community to completely dismiss the knowledge and wisdom
acquired by that community.  Doesn't that disturb you a bit?

> The syntax included in Python is excellent for most things, and even
> at its weakest, is still good. I can't think of any part of Python's
> syntax that is out-and-out bad.

The proposed syntax for using the proposed predicate-based multimethod
library is ungainly.

Until decorators were added to the language, the way to do things that
decorators are good for was ugly.  Decorators patch up one ugliness,
but who wants Python to become an old boat with lots of patches?

Nearly every addition made to Python since 1.5 could have been done in
the standard library, rather than being made to the core language, if
Python had a good macro system.  The exceptions, I think, being
objects all the way down, and generators.  Though generators could
have been done in the standard library too, if Python had first class
continuations, like Scheme and Ruby do.

Over time, an infinite number of examples will turn up like this, and
I claim (1) that it is better to modify the standard library than to
modify the language implementation, and that (2) it is better to allow
people to experiment with language features without having to modify
the implementation, and (3) that it is better to allow people to
distribute new language features for experimentation or production in
a loadable modular fashion, and (4) that it is better to allow
application developers to develope new language features for their
application frameworks than to not.

> The reality is, one can go a long, long, long distance with Python's
> syntax.

And you can go a long, long way with Basic, or Fortran, or C, or C++,
or Haskell, or Lisp.  None of this implies that there aren't
deficiencies in all of these languages.  Python is no exception.
Python just happens to be better than most in a number of significant
regards.

> Most requests for "new syntax" I've seen fall into a few
> categories:

> * optimization, e.g. case, repeat, multi-line lambda

I don't give a hoot about case or repeat, though a Lisp-like "loop
macro" might be nice.  (The loop macro is a little mini language
optimized for coding complicated loops.)  A multi-line lambda would
be very nice.

> * "language Foo looks like this, it is kewl"

Sometimes language Foo has features that are actually important to for
a specific application or problem domain.  It's no accident, for
instance, that Lisp is still the preferred language for doing AI
research.  It's better for Python if Python can accommodate these
applications and domains than for Python to give up these markets to
Foo.

> * the usual braces/whitespace flamewars
> * trying to get static type checking into the language
>
>
> So let's be specific -- what do you think Python's syntax is missing? If
> Python did have a macro facility, what would you change?

In addition to the examples given above, symbols would be nice.  Lisp
has 'em, Ruby has 'em, Python doesn't.  They are very useful.

An elegant multimethod based object system will be essential
for every language someday, when the time is right for people to
understand the advantages.

Manifest typing will be essential.

A backtracking system is important for some applications.  Perhaps all
applications, someday.

The ability to make mini-languages for specific domains, like fields
of math and science, is very useful, so the mathematicians and
scientists can denote things in a notation that is closer to the
notation that they actually work in.

Etc., etc., etc.  The future is long, and our ability to peer into it
is blurry, and languages that can adapt to the unforeseen needs of that
blurry future are the ones that will survive.

For instance, I can state with almost 100% certainty that one hundred
years from now, some dialect of Lisp will still be around and in
common usage.  I can't say the same thing about Python

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> On Sat, 23 Jun 2007 14:56:35 -0400, Douglas Alan wrote:
>
>>> How long did it take you to write the macros, and use them, compared
>>> to running Pylint or Pychecker or equivalent?

>> An hour?  Who cares?  You write it once and then you have it for the
>> rest of your life.  You put it in a widely available library, and then
>> *every* programmer also has it for the rest of their lives.  The
>> amortized cost: $0.00.  The value: priceless.

> Really? Where do I download this macro? How do I find out about it? How
> many Lisp programmers are using it now?

(1) I didn't have to write such a macro for Lisp, as Lisp works
differently.  For one thing, Lisp already has let and set special
forms.  (Lisp uses the term "special form" for what Python would call
a "statement", but Lisp doesn't call them statements since they return
values.)

(2) You act as if I have no heavy criticisms of Lisp or the Lisp
community.  I critique everything with equal vigor, and keep an eye
out for the good aspects and ideas of everything with equal vigor.

> How does your glib response jib with your earlier claims that the
> weakness of Lisp/Scheme is the lack of good libraries?

(1) See above. (2) My response wasn't glib.

> Googling for ' "Douglas Allen" download lisp OR scheme ' wasn't very
> promising.

(1) You spelled my name wrong.  (2) I haven't written any libraries
for any mainstream dialects of Lisp since there was a web.  I did
write a multiple dispatch lookup cacher for a research dialect of
Lisp, but it  was just an exercise for a version of Lisp that few
people have ever used.

> In fairness, the various Python lints/checkers aren't part of the standard
> library either, but they are well-know "standards".

In general I don't like such checkers, as I tend to find them more
annoying than useful.

>> Thanks, but that's just too syntactically ugly and verbose for me to
>> use.

> "Syntactically ugly"? "Verbose"?

> Compare yours with mine:

> let x = 0
> let y = 1
> let z = 2
> set x = 99 

> (Looks like BASIC, circa 1979.)

It looks like a lot of languages.  And there's a reason for that -- it
was a good idea.

> variables.declare(x=0, y=1, z=2)
> variables.x = 99

> (Standard Python syntax.)

> I don't think having two easily confused names, let and set is an
> advantage,

Let and set are not easily confused.  Lisp programmers have had
absolutely no problem keeping the distinction separate for the last 47
years now.

> but if you don't like the word "declare" you could change it to
> "let", or change the name of the module to "set" (although that runs the
> risk of confusing it with sets).

> Because this uses perfectly stock-standard Python syntax, you could even
> do this, so you type fewer characters:

> v = variables
> v.x = 99

> and it would Just Work. 

I wouldn't program that way, and no one that I know would either.

In this regard you sound exactly like all the C++ folks, who when you
point out that something in C++ is inadequate for one's needs, they
point you at some cumbersome and ugly solution and then tell you that
since C++ can already deal with the complaint, that there's no good
reason to consider changing C++.  Consequently, C++ still doesn't have
a "finally" statement, and it requires either making instance
variables public or forcing the programmer to write lots of
boilerplate code writing setter and getter functions.  Fortunately,
the Python developers finally saw the errors of their ways in this
regard and fixed the situation.  But, it seems to me that you would
have been one of those people saying that there's no need to have a
way of overriding attribute assignment and fetching, as you can always
just write all that extra boilerplate code, or instead add an extra
layer of indirection (proxy objects) in your instance data to have
things done the way you want, at the expense of ugly code.

>> Not only that, but my fellow Python programmers would be sure to
>> come and shoot me if I were to code that way.

> *shrug* They'd shoot you if you used "let x = 0" too.

Clearly you are not familiar with the programmers that I work with.
As I mentioned previously, at least one of them is quite upset about
the auto-declaration feature of most scripting languages, and your
suggestion would not make her any happier.

>> One of the reasons that I want to use Python is because I like reading
>> and writing code that is easy to read and looks good.  I don't want to
>> bend it to my will at the expense of ugly looking code.

> But the "ugly looking code" is stock-s

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> But if you really want declarations, you can have them.
>
 import variables
 variables.declare(x=1, y=2.5, z=[1, 2, 4])
 variables.x = None
 variables.w = 0
> Traceback (most recent call last):
>   File "", line 1, in 
>   File "variables.py", line 15, in __setattr__
> raise self.DeclarationError("Variable '%s' not declared" % name)
> variables.DeclarationError: Variable 'w' not declared

Oh, I forgot to mention that I work a lot on preexisting code, which I
am surely not going to go to all the effort to retype and then retest.
With the "let" and "set" macros I can use "set" without a matching
"let".  "set" just checks to make sure that a variable already exists
before assigning to it, and "let" just prevents against
double-declarations.  They can be used independently or together.
With your "variables" class, they have to be used together.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

>> So one use for macros would be so that I can define "let" and "set"
>> statements so that I might code like this:
>> 
>>  let longVariableName = 0
>>  set longVarableName = foo(longVariableName)
>> 
>> Then if longVarableName didn't already exist, an error would be
>> raised, rather than a new variable being automatically created for me.

> So "let" is the initial declaration, and "set" modifies the existing
> variable? 

Yes.

> What happens is you declare a variable twice?

The same thing that would happen in Perl or any other language that
supports this type of variable declaration and setting: it would raise
an error.

The big debate you get next, is then whether you should be allowed to
shadow variables in nested scopes with new variables of the same
name.  Given that Python already allows this, my guess is that the
answer should be yes.

> How long did it take you to write the macros, and use them, compared
> to running Pylint or Pychecker or equivalent?

An hour?  Who cares?  You write it once and then you have it for the
rest of your life.  You put it in a widely available library, and then
*every* programmer also has it for the rest of their lives.  The
amortized cost: $0.00.  The value: priceless.

> But if you really want declarations, you can have them.

 import variables
 variables.declare(x=1, y=2.5, z=[1, 2, 4])
 variables.x = None
 variables.w = 0
> Traceback (most recent call last):
>   File "", line 1, in 
>   File "variables.py", line 15, in __setattr__
> raise self.DeclarationError("Variable '%s' not declared" % name)
> variables.DeclarationError: Variable 'w' not declared

Thanks, but that's just too syntactically ugly and verbose for me to
use.  Not only that, but my fellow Python programmers would be sure to
come and shoot me if I were to code that way.

One of the reasons that I want to use Python is because I like reading
and writing code that is easy to read and looks good.  I don't want to
bend it to my will at the expense of ugly looking code.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Michele Simionato <[EMAIL PROTECTED]> writes:

> Been there, done that. So what? Your example will not convince any
> Pythonista.

I'm a Pythonista, and it convinces me.

> The Pythonista expects Guido to do the language job and the
> application developer to do the application job.

I'm happy to hear that there is a brain washing device built into
Python that provides all Python programmers with exactly the same
mindset, as that will certainly aid in having a consistent look and
feel to all Python code.

> Consider for instance generators.

Yes, consider them!  If Python had first class continuations (like
Ruby does) and macros in 1991, it could have had generators in 1992,
rather than in 2002.  (I implemented generators using macros and stack
groups for Lisp Machines in 1983, and it took me all of a few hours.)

> In Python they are already implemented in the core language and the
> application developer does not care at all about implementing them.

And if they were implemented as macros in a library, then the
application developer doesn't have to care about implementing them
either.

> In Scheme I am supposed to implement them myself with continuations,
> but why should I do that, except as a learning exercise?

Well, when I get around to giving my sage advice to the Scheme
community, I'll let them know that generators need to be in the
standard library, not a roll-your-own exercise.

> It is much better if competent people are in charge of the very low
> level stuff and give me just the high level tools.

Even many competent people don't want to hack in the implementation
language and have to understand the language implementation internals
to design and implement language features.  By your argument,
Pythonistas might as well insist that the entire standard library be
coded in C.

> BTW, there are already Python-like languages with macros
> (i.e. logix) and still nobody use them, including people with a
> Scheme/Lisp background. That /should be telling you something.

It only tells me what I've known for at least a couple decades now --
that languages live and die on issues that often have little to do
with the language's intrinsic merits.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> Nevertheless, in Python 1+2 always equals 3. You can't say the same thing
> about Lisp.

Well, I can't say much of *anything* about "1 + 2" in Lisp, since
that's not the syntax for adding numbers in Lisp.  In Lisp, numbers
are typically added using the "+" function, which might be invoked
like so:

   (+ 1 2 3)

This would return 6.

It's true that some dialects of Lisp will let you redefine the "+"
function, which would typically be a bad idea.  Other dialects would
give you an error or a warning if you tried to redefine "+".  I would
fall more into the latter camp.  (Though sometimes you might want a
way to escape such restrictions with some sort of "yes, I really want
to shoot myself in the head" declaration, as you may want to
experiment, not with changing the meaning of "(+ 1 2"), but rather
with adding some additional useful capability to the "+" function that
it doesn't already have.

Back on the Python front, although "1 + 2" might always equal 3 in
Python, this is really rather cold comfort, since no useful code would
ever do that.  Useful code might include "a + 1", but since you can
overload operators in Python, you can say little about what "a + 1"
might do or mean on the basis of the syntax alone.

Furthermore, in Python you can redefine the "int" data type so that
int.__add__ does a subtraction instead.  Then you end up with such
weirdness as

   >>> int(1.0) + int(2.0)
   -1

Also, you can redefine the sum() function in Python.

So, we see that Python offers you a multitude of ways to shoot
yourself in the head.

One of the things that annoys me when coding in Python (and this is a
flaw that even lowly Perl has a good solution for), is that if you do
something like

 longVarableName = foo(longVariableName)

You end up with a bug that can be very hard to track down.  So one use
for macros would be so that I can define "let" and "set" statements so
that I might code like this:

 let longVariableName = 0
 set longVarableName = foo(longVariableName)

Then if longVarableName didn't already exist, an error would be
raised, rather than a new variable being automatically created for me.

The last time I mentioned this, Alex Martelli basically accused me of
being an idiot for having such trivial concerns.  But, ya know -- it
isn't really a trivial concern, despite Martelli's obviously high
intellect.  A woman I work with who is bringing up a CMS using Drupal
was complaining to me bitterly that this very same issue in PHP was
causing her bugs that were hard to track down.  Unfortunately, I could
not gloat over her with my Python superiority, because if Drupal were
written in Python, rather than PHP, she'd have the very same problem
-- at least in this regard.

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Terry Reedy" <[EMAIL PROTECTED]> writes:

> "Douglas Alan" <[EMAIL PROTECTED]> wrote in message 

> | "Terry Reedy" <[EMAIL PROTECTED]> writes:

> | > I think this points to where Sussman went wrong in his footnote
> | > and Alan in his defense thereof.  Flexibility of function --
> | > being able to do many different things -- is quite different
> | > from flexibility of syntax

> | I think you are setting up a false dichotomy.

> I think this denial of reality is your way of avoiding admitting, perhaps 
> to yourself, that your god Sussman made a mistake.

Sussman isn't my god -- Kate Bush is.

Just because I'm right and you're wrong, doesn't mean that I'm in
denial.  It is you who are in denial if you believe that syntax is
unimportant, as long as one is provided the required functionality.
In fact, that's stereotypical Computer Science denial.  Computer
Science academics will typically state as a truism that semantics are
what is important and syntax is just a boring trifle in comparison.
But time and time again, you'll see programming languages succeed or
fail more on their syntax than on their semantics.  And issues of
syntax is often where you see the most inflamed debates.  Just look at
all the flames one used to hear about Python using whitespace
significantly.  Or all the flames that one will still hear about Lisp
using a lot of parentheses.

You seem oblivious to the fact that one of the huge benefits of Python
is its elegant and readable syntax.  The problem with not having a
"flexible syntax", is that a programming language can't provide
off-the-shelf an elegant syntax for all functionality that will ever
be needed.  Eventually programmers find themselves in need of new
elegant functionality, but without a corresponding elegant syntax to
go along with the new functionality, the result is code that does not
look elegant and is therefore difficult to read and thus maintain.

Consequently, "flexibility of function" is often moot without
"flexibility of syntax".  I don't know how I can make it any clearer
than this.  I'm sorry if you don't understand what I am saying, but
just because you don't understand, or if you do, that you don't agree,
doesn't mean that I don't have a reasoned and reasonable point of
view.

> | One that is related to the false unification that annoying people
> | used to always make when they would perpetually argue that it
> | wasn't important which programming language you programmed in, as
> | they are all Turing equivalent anyway.  Well, I sure as hell don't
> | want to write all my programs for a Turning machine, and a Turing
> | machine is certainly Turing equivalent!

> Diversionary crap unrelated to the previous discussion.

Take the issue up with Paul Graham.  Since making a fortune developing
software in Lisp (making heavy use of macros), he now has much more
free time to write essays defending the truth than I do:

   http://www.paulgraham.com/avg.html

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Terry Reedy" <[EMAIL PROTECTED]> writes:

> | But why is the ability to abstract syntax good?

> I think this points to where Sussman went wrong in his footnote and
> Alan in his defense thereof.  Flexibility of function -- being able
> to do many different things -- is quite different from flexibility
> of syntax 

I think you are setting up a false dichotomy.  One that is related to
the false unification that annoying people used to always make when
they would perpetually argue that it wasn't important which
programming language you programmed in, as they are all Turing
equivalent anyway.  Well, I sure as hell don't want to write all my
programs for a Turning machine, and a Turing machine is certainly
Turing equivalent!

Functionality is no good if it's too cumbersome to use.  For instance,
Scheme gives you first class continuations, which Python doesn't.
Continuations let you do *all sorts* of interesting things that you
just cannot do in Python.  Like backtracking, for instance.  (Well
maybe you could do backtracking in Python with lots of putting code
into strings and liberal use of eval, for all I know, but the results
would almost certainly be too much of a bear to actually use.)

Now, continuations, by themselves, in Scheme actually don't buy you
very much, because although they let you do some crazy powerful
things, making use of them to do so, is too confusing and verbose.  In
order to actually use this very cool functionality, you need macros so
that you can wrap a pretty and easy-to-use face on top of all the
delicious continuation goodness.

You'll, just have to trust me on this one.  I've written code with
continuations, and I just couldn't make heads or tails out of the code
a few hours later.  But when prettied-up with a nice macro layer, they
can be a joy to behold.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> On Thu, 21 Jun 2007 15:25:37 -0400, Douglas Alan wrote:

>> You are imagining something very different from what is proposed.
>> Lisp-like macros don't allow "anything goes".

> Provided people avoid doing anything "which would be considered very 
> rude" (your own words).

No, Lisp macros are entirely contained within a begin and end
delimiter, which is introduced by the name of the macro.  E.g., here
is a real example of some Lisp code that I wrote aeons ago using the
loop macro:

   (loop for index from 0 below size
  for element in contents
  do (store (arraycall t array index)
element)
  finally (return (make-htable BUCKETS size
   ARRAY array
   KEY-PRINTER key-printer
   ITEM-PRINTER item-printer)

The syntactical extensions supported by the loop macro can only begin
starting immediately after "(loop " and they end with the matching
closing parenthesis.  There's no way in Lisp to write a macro whose
syntactical extensions can extend outside of this very explicitly
delimited scope.  Nor can they mess with Lisp's tokenization.

> Python already allows me to shoot myself in the foot, if I wish. I'm 
> comfortable with that level of freedom. I'm not necessarily comfortable 
> with extensions to the language that would allow me the freedom to shoot 
> myself in the head.

Lisp macros don't let you shoot yourself in the head -- only in the
foot.  Being able to do

   True = False

is being able to shoot yourself in the head.  And Python certainly
lets you do that.

> I would need to be convinced of the advantages, as would many other
> people, including the BDFL.

The proof is in the pudding for anyone who has seen the advantages it
brings to Lisp.  As Paul Graham points out, it's hard to look up and
see the advantages of what is up there in a more powerful language.
It's only easy to look down and see the disadvantages of what is
missing from a less powerful language.  To understand the advantages,
one has to be willing to climb the hill and take in the view.

> It isn't clear exactly what functionality a hypothetical Python macro 
> system would include,

It should be largely equivalent to what is provided by Lisp.
Admittedly this is a bit more difficult for Python, as Lisp's syntax
is eminently suited for macros, while Python's is not.  One would
probably want to take a look at how Dylan solved this problem, as
Dylan implements Lisp-like macros even though it has an Algol-like
syntax.  Or you could look at the paper I wrote (for a class) on the
design of Python-like language that would support macros.  My paper is
only a rough sketch, however.

> let alone whether the benefits would outweigh the costs,

They pay off in boatloads in the Lisp community.

> (It took Lisp half a century and millions of dollars of corporate
> funding to reach where it is now.

Ummm, not really.  Lisp hasn't really changed very much since the late
'70s, and prior to that, most of the work on Lisp was just done in a
few university labs (e.g., MIT) and at Xerox Parc.  Any work and money
that has been spent on Lisp since then has just been in trying to
market it, or standardize it, or design hardware suited to running it
faster, or build better development environments for it, or optimizing
compilers, etc.

Lisp, itself, is rather easily to implement.  (Getting it to run as
fast as C is more of a challenge, what with good garbage collectors
and all being non-trivial to implement, but performance doesn't seem
to be much of an issue for the Python community.)  I made my own Lisp
implementation in C++ in two weeks.  (Not really a production dialect,
but it worked.)  Kyoto Common Lisp, which was definitely a production
implementation, was implemented by two people in a couple of years.
(It compiled Common Lisp into C.)

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Terry Reedy" <[EMAIL PROTECTED]> writes:

> "Douglas Alan" <[EMAIL PROTECTED]> wrote in message 

> | > But why is the ability to abstract syntax good?

> | It allows the community to develop language features in a modular way
> | without having to sully the code base for the language itself.

> Anyone can write modules, experimental or otherwise, without touching the 
> code base for any particular implementation.

> For those whose know one of the implementation languages, source code 
> control systems allow one to do experiments on branches without 'sullying' 
> the trunk or impeding the development thereof.

When I said "without having to sully the code base", I meant that one
can implement a language feature for the target language as a loadable
module written entirely within the language itself, and without having
to understand anything particularly deep or specific about the language
implementation details.

I.e., I could write a new object system for Lisp faster than I could
even begin to fathom the internal of CPython.  Not only that, I have
absolutely no desire to spend my valuable free time writing C code.
I'd much rather be hacking in Python, thank you very much.

> One of the goals of the PyPy project was to allow people to experiment with 
> syntax extensions in Python itself.  (But I don't know how easy that is 
> yet.)

PyPy sounds like a very interesting project indeed!

> But I think that overall the problem of designing new syntax is more
> in the design than the implementation.  Anything new has to be
> usable, readable, not clash too much with existing style, not
> introduce ambiguities, and not move the extended language outside
> the LL(1) [I believe that is right] subset of CFLs.

People (myself included) haven't had much trouble implementing nice
and useful macro packages for Lisp.  Admittedly, it's a harder problem
for a language that doesn't have a Lisp-like syntax.  I believe that
Dylan has macros without having a Lisp-like syntax, but Dylan is
really a dialect of Lisp, only with a more traditional Algol-like
syntax veneered onto it.  My guess is that a macro developer for Dylan
would have to be familiar with an underlying hidden intermediate Lisp
syntax.  (Though I'm just really just spouting that guess out of my
butt.)

A few years back, I designed a somewhat Python-like language with a
macro facility for a class on dynamic languages and their
implementations.  I didn't implement it, however, and I doubt that
I'll have time to get around to it in this lifetime.

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Neil Cerutti <[EMAIL PROTECTED]> writes:

> That said, I wouldn't give up the summer I spent studying _Simply
> Scheme_.

Sounds like fun.  Is this like a kinder, gentler version of SICP?

I'm not sure, though, that I could have learned computer science
properly without the immortal characters of Ben Bittwiddler and Harry
Reasoner intruding into every problem set.

|>oug
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

"Terry Reedy" <[EMAIL PROTECTED]> writes:

> | It allows the community to develop language features in a modular way
> | without having to sully the code base for the language itself.
> [etc]

> Some of the strongest opposition to adding macros to Python comes
> from people like Alex Martelli who have had experience with them in
> *multi-person production* projects.  He claimed in various posts
> that the net effect was to reduce productivity.  So convince us (and
> Guido!) that he is wrong ;-)

I'm not convinced that Guido is wrong because I know that he has at
least occasionally mused that he might someday consider a macro
facility for Python.

Alex Martelli, on the other hand, although an extremely smart guy,
seems to me to often be over-opinionated and dismissive.

Regarding being on a project where people used macros poorly, I've
also been on projects where people did a poor job of OO design, and a
non-OO design would have been better than the crappy OO design that
was ultimately used.  Does that mean that we should remove the OO
features from Python?

Paul Graham made it rich implementing Yahoo Stores in Lisp, and claims
that heavy use of macros is one of the reasons that he was able to
stay well-ahead of all the competition.  So, maybe Paul and Alex can
duke it out.  Personally, I like Paul's style better.  And he's made a
lot more money using his theory of software design.

> But I would prefer you somehow try to help make usable multi-arg and 
> predicate dispatch a reality.

Alas, I can't stand programming in C, so there's no way I'm going to
dive that deeply into the CPython code base.  If I could help
implement it in Python itself, using a good macro facility, sign me up!

|>oug
-- 
http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Neil Cerutti <[EMAIL PROTECTED]> writes:

>>> But why is the ability to abstract syntax good?

>> It allows the community to develop language features in a
>> modular way without having to sully the code base for the
>> language itself.  

> That's not an advantage exclusive to macros, though.

No, but macros are often are necessary to be able to implement such
features in (1) an efficient-enough manner, and (2) in a manner that
is syntactically palatable.  E.g., PEAK for Python implements multiple
predicate-based dispatch, but you have to define the predicates as
Python code within strings.  That's not very pretty.  And probably not
very fast either.  Though Python, in general, is not very fast, so
perhaps that doesn't matter too much for Python.

> Some time last week I found myself writing the following thing in
> Python:

> [...]

> I deleted it right after I tried to use it the first time. Using it
> is more cumbersome than simply repeating myself, due to syntax
> limitations of Python.

See what I mean!

> And other, more bizarre syntax extensions have been perpetrated.
> mx.TextTools uses Python tuples to write a completely different
> programming language.

Sounds like "the Loop macro" for Lisp, which implements a mini sort of
Cobol-like language just for coding gnarly loops within Lisp.  It
turns out that when restricted to just coding gnarly loops, this is a
much better idea than it sounds.

Yes, you can do this sort of thing, sort of, without macros, but, as
we discussed above, the result is often ugly and slow.

>> A prime example of this is how CLOS, the Common Lisp Object
>> System was implemented completely as a loadable library (with
>> the help of many macros) into Common Lisp, which was not an OO
>> language prior to the adoption of CLOS.

> Is there a second example? ;)

Why yes, now that you mention it: the Loop macro.  Also, in many
implementations of Lisp, much of the core language is actually
implemented using macros against an even smaller core.  Keeping this
inside core as small as possible helps make the implementation easier
to construct, maintain, and optimize.

Also, way back when, when I used to code in Maclisp, I implemented my
own object system and exception handling system in macros, as Maclisp
had neither of these off the shelf.  The object system took me a
couple of weeks to do, and the exception handing system a couple of
days.  They worked well, looked good, and ran fast.

> Seriously, maybe Python looks like 'blub' (thanks, Paul Graham), to
> the skilled Lisp user, but it makes a lot of other languages look
> like 'blub', too, including, sometimes, Lisp: Lisp has to 'blub'
> generators.

Actually, Scheme has first class continuations, and with continuations
and macros you could easily implement generators, and I'm sure someone
has.  Whether such a library has been widely adopted for Scheme,
though, I have no idea.

You're probably right about Common Lisp, which is probably missing
generators due to efficiency concerns.  Lisp Machines had "stack
groups", which were basically the same thing as generators, but making
a call to a stack group was 100 times slower than a normal function
call.  This meant that people generally didn't use them even when it
would make their code more elegant, due to the huge performance cost.

Now, since Python is like 100 times slower than Common Lisp anyway,
you don't notice this performance issue with Python's generators.
They just happen to be only as slow as the rest of Python.

|>oug

"Lisp is worth learning for the profound enlightenment experience you
will have when you finally get it; that experience will make you a
better programmer for the rest of your days, even if you never
actually use Lisp itself a lot." -- Eric Raymond
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Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> On Wed, 20 Jun 2007 17:23:42 -0400, Douglas Alan wrote:

>> Macros are a way to abstract syntax the way that objects are used to
>> abstract data types and that iterators and generators abstract control,
>> etc.

> But why is the ability to abstract syntax good?

It allows the community to develop language features in a modular way
without having to sully the code base for the language itself.  A
prime example of this is how CLOS, the Common Lisp Object System was
implemented completely as a loadable library (with the help of many
macros) into Common Lisp, which was not an OO language prior to the
adoption of CLOS.

The fact that CLOS could be developed in a modular way allowed for a
number of different groups to work on competing object systems.  After
some experience with the pros and cons of the various object systems,
the developers of CLOS were able to incorporate most of the best ideas
from the entire field and then get it adopted as a defacto standard.

This allowed, for instance, the inclusion of multimethods, which are
an extremely nice feature for modular code development.  In prior Lisp
dialects I had used, the object systems were more like the
single-object dispatching OO system in Python, which is substantially
inferior.  The fact that the entire OO system in Common Lisp could be
loaded as a module that is coded entirely within Common Lisp allowed
for a large jump in the quality of its OO subsystem.

> One criticism of operator overloading is that when you see X + Y you
> have no real idea of whether it is adding X and Y together, or doing
> something bizarre.

Yes, and despite this, operator overloading is an essential feature
for a modern language.  (Java's lack of it notwithstanding.)

> Now allow syntax to be over-ridden as well, and not only can't you tell 
> what X + Y does, but you can't even tell what it *means*. Maybe its a for-
> loop, calling the function Y X times.

(1) With operator overloading you have no idea what X + Y *means*.  It
could be feeding the cat, for all you know.  If that turns out to
be the case, you fire the programmer in question.  Just because a
language feature *can* be abused is no reason to leave it out of a
language.  Power always comes with responsibility, but we still
need powerful programming languages.

(2) In Lisp, you cannot redefine existing syntax (without modifying
the standard library, which would be considered very rude), so the
problem that you are talking about is moot.  You can only add
*new* syntactic constructs.  I would suggest that any proposed
syntax extension mechanisms for other languages behave like Lisp
in this regard.

> Sometimes, more freedom is not better. If you had a language that
> let you redefine the literal 1 to mean the integer zero, wouldn't
> that make it much harder to understand what even basic arithmetic
> meant?

But we're not talking about anything like this.  E.g., in some
dialects of Lisp it used to be possible to set the variable that
contained the value for *true* to the value for *false*.  If you
actually did this, however, just imagine the havoc that it would
wreak.  So ultimately, this capability was removed.

Alas, in Python, you can still do such a crazy thing!

> But that doesn't mean I want a language where anything goes

You are imagining something very different from what is proposed.
Lisp-like macros don't allow "anything goes".

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Robert Kern <[EMAIL PROTECTED]> writes:

>> The problem with Python's model is that you
>> have to wait for a rather centralized process to agree on and
>> implement such a feature.

> No, you don't. Philip Eby has been working on various incarnations
> of generic functions for some time now. The only thing new with 3.0
> is that they may be included in the standard library and parts of
> the rest of the standard library may use them to implement their
> features. Implementing generic functions themselves don't require
> anyone to convince python-dev of anything.

>   http://python.org/pypi/simplegeneric/0.6
>   http://peak.telecommunity.com/DevCenter/RulesReadme

The first one doesn't do multiple dispatch.  I'll have to have a look
at the second one.  It looks interesting.  This link

   http://www.ibm.com/developerworks/library/l-cppeak2/

shows PEAK being used to do multiple dispatch based on predicates, but
the code to implement the predicates is in strings!  Sure, if you
don't have macros, you can always use eval instead if you have it, but
(1) doing so is ugly and dangerous, and (2) it's inefficient.  Lisp
implementations these days do fancy stuff (multiple dispatch, etc.)
implemented as macros and yet typically end up generating code that
runs within a factor of 2 of the speed of C code.

In addition to having to code predicates in strings, using PEAK seems
syntactically rather cumbersome.  Macros would help solve this
problem.  Decorators seem to help to a large extent, but they don't
help as much as macros would.

Thanks for the pointers.

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Michele Simionato <[EMAIL PROTECTED]> writes:

> In practice Scheme follows exactly the opposite route: there are
> dozens of different and redundant object systems, module systems,
> even record systems, built just by piling up feature over feature.

The solution to this is to have a standard library which picks the
best of each and standardizes on them.  (E.g., for Common Lisp, CLOS
became the standard object system, but there was certainly competition
for a while.  E.g., Flavors, Common Loops, etc.) The problem with this
for Scheme is that the Scheme standardizing committees operate at a
glacial pace.

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> All of which makes Douglas Alan's accusations of Not Invented Here 
> syndrome about Python seem rather silly.

I've never made such an accusation about Python itself -- just about
the apparent attitude of some pontiffs.

> The point I was making isn't that Scheme/Lisp features are "bad",
> but that there is no reason to slavishly follow Scheme just because
> it is(?)  technically the most "pure" programming language.

I don't argue in favor of purity.  I argue in favor of functionality
that would help me write better programs more easily.

Having a somewhat pure and minimalistic core language, however,
probably does help to make a language easier to implement, maintain,
and understand.

> I'm glad somebody understands lambda calculus and closures and meta-
> classes, and that those people have created Python so I don't have
> to.  And I suspect that for every Douglas Alan enamored with Scheme,
> there are ten thousand programmers who just want to use a handful of
> pre-built tools to get the work done, never mind using macros to
> create the tools they need before they can even start.

I don't typically want to write that many macros myself.  I want to be
part of a community where cool macro packages are actively developed
that I can then use.  For instance, in Common Lisp, the entire OO
system is just a big macro package that was implemented entirely
within the language.

With macros and first class continuations, as Sussman points out in
his essay, people can then implement very interesting features like
backtracking.  Again, I don't want to implement a backtracking macro
package myself; I want to be able to use what the community might come
up with.

> But "Scheme has macros" isn't a justification for why Python should
> have them.

No one ever gave that justification.  The justification is that they
are *good*.

Macros are a way to abstract syntax the way that objects are used to
abstract data types and that iterators and generators abstract
control, etc.

|>oug
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http://mail.python.org/mailman/listinfo/python-list

Re: Python's "only one way to do it" philosophy isn't good?

[Douglas Alan]

Steven D'Aprano <[EMAIL PROTECTED]> writes:

> On Tue, 19 Jun 2007 20:16:28 -0400, Douglas Alan wrote:

>> Steven D'Aprano <[EMAIL PROTECTED]> writes:

>>> On Tue, 19 Jun 2007 17:46:35 -0400, Douglas Alan wrote:

>> The problem with using Scheme for real work is that it doesn't come
>> with enough batteries included and there isn't a big enough of a
>> community behind it that uses it for real work.

> And yet there have been so many millions of dollars put into
> developing Lisp...

> I guess this is another example of perfection being the enemy of the
> good.

That's probably a valid criticism of Scheme.  Not so much of Lisp in
general.  The reason that Common Lisp hasn't been widely adopted
outside of the AI community, for instance, has more to do with most
programmers apparently not understanding the joys of Cambridge Polish
Notation.  Also, Lisp was garbage-collected back when
garbage-collection had a bad name, due to it being considered a
resource hog.  And then a large part of the Lisp community decided to
concentrate on building special hardware (i.e. Lisp machines) for
developing Lisp applications, rather than making good Lisp development
environments for Fortran/C machines (i.e., all normal computers).

It was clear to me at the time that it's a very hard sell to convince
people to buy expensive hardware to develop in a programming language
that hasn't yet been popularized.  But apparently what was obvious to
me was not so apparent to those who dreamed of IPO riches.

> All that development into Lisp/Scheme to make it the best, purest,
> most ideal programming language, with such flexibility and
> extensibility.  that nobody wants to use it. You can write any
> library and macro system you need, but nobody has.

Lisp in general has had all sorts of fancy macro packages for it since
the dawn of time.  But Lisp wasn't really standardized until Common
Lisp in the early '80s.  CLOS (the Common Lisp Object System), which
is implemented via macros entirely within Common Lisp itself, was
completed in the latter half of the '80s.

> I don't mean literally nobody, of course. Its a figure of
> speech. But it seems that people tend to program in Scheme for fun,
> or to stretch the boundaries of what's possible, and not to Get The
> Job Done.

Well, most implementations don't have batteries included, the way that
Python does.  Guile attempts to, as does the Scheme Shell, but for
various reasons they didn't catch on the way that Python and Perl and
Ruby have.

>> Well, go to MIT and take SICP and then the graduate-level sequel to
>> the class, Adventures in Advanced Symbolic Programming, and then
>> you'll see what some of the advantages would be.

> Are you suggesting that the only way to see the advantages of Scheme
> is to do a university course?

No, I was just suggesting *one* way of seeing the light.  I think,
though, that it's probably difficult to grasp why Scheme is so
interesting without taking a good class or two on the topic.  Not that
acquiring insight on one's own is impossible, but most people would
also have a hard time seeing why group theory or linear algebra are
really interesting without taking good classes one the subjects.

Python doesn't really require taking a class in it because it's really
very similar to many other programming languages.  So if you already
know any of the others, Python is very easy to pick up.  Scheme, on
the other hand, is more of a departure from what most people would
already be comfortable with.

>> A good multimethod system, e.g., would make Python a significantly nicer
>> language for my purposes, for instance.

> http://en.wikipedia.org/wiki/Multimethod#Python

Sure, you can do it in Python, but I bet that it's neither very fun,
nor efficient.

>> For the record, I have a huge problem with NIH-syndrome, and think that
>> every programming language in the world could learn a thing or two from
>> what other languages have gotten right.

> Of course. And Python, more than most, has shamelessly copied
> features from other languages.

To make myself a bit more clear, I don't think that Python suffers all
that much from NIH.  On the other hand, I think that many people in
discussion forums in general often do.  Here being no exception.

> So the question is, are Scheme macros one of those things that
> "other languages have gotten right"? Could they be a case of
> over-generalization? Or somewhere in between?

I don't know of any language that is not a dialect of Lisp that has a
good syntax extension mechanism.  Lisp dialects tend to get it right,
more or less, but solving the issue is much more difficult for any
language that doesn't have a Lisp-like syntax.  Lisp's syntax is
particularly amenable to syntax extension.

|>oug
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