Problem with Lexical Scope

[[EMAIL PROTECTED]]

I am not completely knowledgable about the status of lexical scoping in
Python, but it was my understanding that this was added in a long time
ago around python2.1-python2.2

I am using python2.4 and the following code throws a "status variable"
not found in the inner-most function, even when I try to "global" it.

def collect(fields, reducer):
def rule(record):
status = True
def _(x, y):
cstat = reducer(x, y)
if status and not cstat:
status = False
return y
return reduce(_, [record[field] for field in fields])
return rule

What gives?

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Re: newbie question

[Tim Roberts]

"Bermi" <[EMAIL PROTECTED]> wrote:

>i have this program
>===
>from sys import *
>import math
>import math, Numeric
>from code import *
>from string import *
>from math import *
>from dataSet import *
>from string import *

I know you said you are a newbie.  That means there is still time for you
to learn to do things the proper way.  Change all of that to:

  import sys
  import math
  import Numeric
  import dataSet

You don't need to import string at all, and you will probably never use
"code".


>def drawAsciiFile():
>_fileName=str(argv[1])

argv is already an array of strings.  The "str" is useless.

>__localdataSet=DataSet(_fileName)
>
>#_PlotCols=string.split(str(argv[2]),' ')
>#_PlotColsInt=[]
>'''for s in _PlotCols:
>_PlotColsInt.append(int(s))
>CountourPlots(__localdataSet,_PlotColsInt)
>'''
>print
>__data=__localdataSet.GetData()
>print max(__data[:,11])
>if __name__ == "__main__":
>drawAsciiFile()
>
>
>how i can link it to read my file examle.txt?

Instead of referring to argv inside your function, you should pass the file
name as a parameter.  That way, at a later time, you can pass it file names
from sources other than argv.  Also, there is no point in using leading
underscores for local variables.  That's only interesting for members of
the class.

You are splitting argv[2].  Ordinarily, sys.argv has already been split.
That is, "xxx.py file 1 3 5 7" will produce 6 elements in sys.argv.  If you
really want the columns as a single parameter, you'll have to use quoting:
xxx.py file "1 3 5 7"

Something like this, maybe:

def drawAsciiFile(filename, columns):
localdataSet = DataSet.DataSet( filename )
PlotCols = [int(s) for s in columns.split()]
ContourPlots( localdataSet, PlotCols )

if __name__ == "__main__":
if len(sys.argv) < 2:
print 'Usage:  .py filename "3 5 7"' )
drawAsciiFile( sys.argv[1], sys.argv[2] )
-- 
- Tim Roberts, [EMAIL PROTECTED]
  Providenza & Boekelheide, Inc.
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Re: lambda (and reduce) are valuable

[Steven Bethard]

Paul Rubin wrote:
> Chris Mellon <[EMAIL PROTECTED]> writes:
> 
>>As someone who does a tremendous amount of event-driven GUI
>>programming, I'd like to take a moment to speak out against people
>>using us as a testament to the virtues of lamda. Event handlers are
>>the most important part of event-driven code, and  making them real
>>functions with real names is crucial to maintainable code. The only
>>reason to ever use a lamdba in Python is because you don't want to
>>give a function a name, and that is just not a compelling use case for
>>GUI events.
> 
> 
> I thought stuff like the following was idiomatic in GUI programming.
> Do you really want separate names for all those callbacks?
> 
> # generate calculator keypad buttons
> Button(label='7', command=lambda: user_pressed(7)).grid(column=1, row=1)
> Button(label='8', command=lambda: user_pressed(8)).grid(column=2, row=1)
> Button(label='9', command=lambda: user_pressed(9)).grid(column=3, row=1)
> 
> Button(label='4', command=lambda: user_pressed(4)).grid(column=1, row=2)
> Button(label='5', command=lambda: user_pressed(5)).grid(column=2, row=2)
> Button(label='6', command=lambda: user_pressed(6)).grid(column=3, row=2)
> ...

While I don't spend much time on GUIs, code like that would scream 
"refactor" to me, e.g. something like:

class UserPressedButton(Button):
 def __init__(self, i):
 def command():
 return user_pressed(i)
 Button.__init__(self, label=str(i), command=command)

Button(7).grid(column=1, row=1)
Button(8).grid(column=2, row=1)
Button(9).grid(column=3, row=1)

Button(4).grid(column=1, row=2)
Button(5).grid(column=2, row=2)
Button(6).grid(column=3, row=2)

STeVe
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Re: puzzled about class attribute resolution and mangling

[Brian van den Broek]

James Stroud said unto the world upon 2005-12-09 20:39:
> Brian van den Broek wrote:
> 
>>Hi all,
>>
>>I've the following code snippet that puzzles me:
>>
>>class Base(object):
>>__v, u = "Base v", "Base u"
>>def __init__(self):
>>print self.__v, self.u
>>
>>class Derived(Base):
>>__v, u = "Derived v", "Derived u"
>>def __init__(self):
>>print self.__v, self.u
>>super(Derived, self).__init__()
>>
>>d = Derived()
>>
>>When run (Python 2.4.2, IDLE 1.1.2), it produces:
>>
>> >>>
>>Derived v Derived u
>>Base v Derived u
>> >>>
>>
>>What I expected was that all four emitted strings would contain "Derived".



>>Thanks and best,
>>
>>Brian vdB
> 
> 
> This is name mangling at work. Mangling turns self.__v in the Derrived 
> class's 
> __init__ method to self._Derrived__v and self.__v in the Base class's 
> __init__ 
> method to self._Base__v. These are different names bound to different values 
> and 
> are reflected as such. self.u is the same name in both cases and the value 
> was 
> bound in the Derrived class, and not re-bound in the Base class.
> 
> James

Thanks for the reply, James. Rereading the relevant section of the
Tutorial:

>> Any identifier of the form __spam (at least two leading
>> underscores, at most one trailing underscore) is textually replaced
>> with _classname__spam, where classname is the current class name
>> with leading underscore(s) stripped.


I'm not sure how I came to think that names were mangled only with
respect to calling code from outside class definitions.

As I'd been confused, I naturally started thinking of ways to clarify
the docs. But, I've come to think that the error was wholly mine.

Thanks for the help,

Brian vdB


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Re: Using XML w/ Python...

[uche . ogbuji]

"""
Spoke too soon, i get this error when running amara in ActivePython

>>> import amara
>>> amara.parse("http://www.digg.com/rss/index.xml";)

Traceback (most recent call last):
  File "", line 1, in ?
  File "C:\Python23\Lib\site-packages\amara\__init__.py", line 50, in
parse
if IsXml(source):
NameError: global name 'IsXml' is not defined

So im guessing theres an error with one of the files...
"""

IsXml is imported conditionally, so this is an indicator that somethign
about your module setup is still not agreeing with ActivePython.   What
do you see as the output of:

python -c "import amara; print dir(amara)"

?  I get:

['InputSource', 'IsXml', 'Uri', 'Uuid', '__builtins__', '__doc__',
'__file__', '__name__', '__path__', '__version__', 'bindery',
'binderytools', 'binderyxpath', 'create_document', 'dateutil_standins',
'domtools', 'os', 'parse', 'pushbind', 'pushdom', 'pyxml_standins',
'saxtools']

--
Uche Ogbuji   Fourthought, Inc.
http://uche.ogbuji.nethttp://fourthought.com
http://copia.ogbuji.net   http://4Suite.org
Articles: http://uche.ogbuji.net/tech/publications/

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Re: nanothreads: Want to use them from within wxPython app

[simonwittber]

F. GEIGER wrote:
> I've def'ed a handler for EVT_IDLE in the app's main frame. There I'd like
> to call the nanothreads' __iter__ method, somehow.
>
> When I copy the __iter__ method into a, say, runOnce() method and call the
> next() method of the generator returned by runOnce(), it works. But I can't
> get at the __iter__ method, which is already there and therefore should be
> used instead of messing up nanothreads with changes of mine.
>
> Any hint welcome

The latest version of nanothreads is now in the fibranet package, which
you can download from the cheeseshop:

http://cheeseshop.python.org/pypi/FibraNet

To iterate nanothreads from wx, I call the nanothreads.poll() function
from the EVT_IDLE handler, making sure that I call event.RequestMore()
from within the handler, to iterate nanothreads as fast as possible.

HTH, Simon WIttber.

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Re: Using XML w/ Python...

[Jay]

Spoke too soon, i get this error when running amara in ActivePython

>>> import amara
>>> amara.parse("http://www.digg.com/rss/index.xml";)
Traceback (most recent call last):
  File "", line 1, in ?
  File "C:\Python23\Lib\site-packages\amara\__init__.py", line 50, in
parse
if IsXml(source):
NameError: global name 'IsXml' is not defined

So im guessing theres an error with one of the files...

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Re: lambda (and reduce) are valuable

[Paul Rubin]

Chris Mellon <[EMAIL PROTECTED]> writes:
> As someone who does a tremendous amount of event-driven GUI
> programming, I'd like to take a moment to speak out against people
> using us as a testament to the virtues of lamda. Event handlers are
> the most important part of event-driven code, and  making them real
> functions with real names is crucial to maintainable code. The only
> reason to ever use a lamdba in Python is because you don't want to
> give a function a name, and that is just not a compelling use case for
> GUI events.

I thought stuff like the following was idiomatic in GUI programming.
Do you really want separate names for all those callbacks?

# generate calculator keypad buttons
Button(label='7', command=lambda: user_pressed(7)).grid(column=1, row=1)
Button(label='8', command=lambda: user_pressed(8)).grid(column=2, row=1)
Button(label='9', command=lambda: user_pressed(9)).grid(column=3, row=1)

Button(label='4', command=lambda: user_pressed(4)).grid(column=1, row=2)
Button(label='5', command=lambda: user_pressed(5)).grid(column=2, row=2)
Button(label='6', command=lambda: user_pressed(6)).grid(column=3, row=2)
...
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Re: Using XML w/ Python...

[Jay]

h, i just tryed the same thing earlier today and it didnt work, but
now it does, i downloaded the standalone package and now it works in
activepython when it didnt before and i tryed the same thing.

And yes, last time i did type python setup.py install.

Thx anyway.

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Re: Using XML w/ Python...

[uche . ogbuji]

"""
No, when i said
 "As far as it should work since their both transparent, umm, well its
not."

I meant that only mine isnt, maybe urs is but for some reason it isnt.
And you said amara works fine for you, ok, then could you tell me what
package to install...

I have installed Amara 1.1.6 for Python 2.4 and it works on python 2.4
only.
Now, which package should i download for it to work on any python
prompt:
  Allinone
  Standalone
  Or something else
"""

I've never used ActivePython.  I don't know of any special gotchas for
it.  But Amara works in Python 2.3 or 2.4.  The only differences
between the Allinone and standalone packages is that Allinone includes
4Suite.  Do get at least version 1.1.6.

If you're still having trouble with the ActivePython setup, the first
thing I'd ask is how you installed Amara.  DId you run a WIndows
installer?  Next I'd check the library path for ActivePython.  What is
the output of

python -c "import sys; print sys.path"

Where you replace "python" abpve with whatever way you invoke
ActivePython.


--
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http://uche.ogbuji.nethttp://fourthought.com
http://copia.ogbuji.net   http://4Suite.org
Articles: http://uche.ogbuji.net/tech/publications/

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Re: how to put form and display its result(data from database) on the same window?

[Tim Roberts]

Peter Hansen <[EMAIL PROTECTED]> wrote:
>
>Well, it's a matter of interpretation, isn't it?  Given his vague 
>requirements above, it's quite valid to assume that he wants to avoid 
>the effects of the browser loading a new page, even if that page looks 
>like the previous one a lot.  You may be quite right in your *guess* 
>that he doesn't need that, but there isn't enough information present in 
>his request for either of us to know.

You are correct.  Once you pointed it out, I now see what you were
suggesting.
-- 
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  Providenza & Boekelheide, Inc.
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Re: lambda (and reduce) are valuable

[David Isaac]

"Chris Mellon" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]

> As someone who does a tremendous amount of event-driven GUI
> programming, I'd like to take a moment to speak out against people
> using us as a testament to the virtues of lamda. Event handlers are
> the most important part of event-driven code, and  making them real
> functions with real names is crucial to maintainable code. The only
> reason to ever use a lamdba in Python is because you don't want to
> give a function a name, and that is just not a compelling use case for
> GUI events.

Obviously opinions differ. See the discussion at
http://www.artima.com/forums/flat.jsp?forum=106&thread=98196&start=30&msRange=15
I find many of the pleas for lambda persuasive as well.
For the contribution of lambda to maintainability,
see e.g. Gary Robinson's comment at that link.

Alan Isaac


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Re: Using XML w/ Python...

[Jay]

No, when i said
 "As far as it should work since their both transparent, umm, well its
not."

I meant that only mine isnt, maybe urs is but for some reason it isnt.
And you said amara works fine for you, ok, then could you tell me what
package to install...

I have installed Amara 1.1.6 for Python 2.4 and it works on python 2.4
only.
Now, which package should i download for it to work on any python
prompt:
  Allinone
  Standalone 
  Or something else

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Re: lambda (and reduce) are valuable

[David Isaac]

> Alan Isaac wrote:
> >>> #evaluate polynomial (coefs) at x using Horner's rule
> >>> def horner(coefs,x): return reduce(lambda a1,a2: a1*x+a2,coefs)
> > It just cannot get simpler or more expressive.


"Peter Otten" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> But is it correct?

Yes.

> Are we merely employing different conventions for the order of
coefficients
> or is that simple and expressive lambda/reduce stuff obscuring an error?

It is too simple and expressive to obscure an error.   ;-)
This is particularly important since coefficient order is not standardized
across uses.

Cheers,
Alan Isaac


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Re: newbie question about python and Tkinter

[James Stroud]

newbie wrote:
[about some tkinter problems]

I'm running:

Python 2.3.4 (#4, Oct 25 2004, 21:40:10)
[GCC 3.3.2 (Mandrake Linux 10.0 3.3.2-6mdk)] on linux2

With tcl/tk 8.3 and your first sample works as expected.

> I am running PYTHON and Tkinter on a windows' XP box.

This is probably your problem. Seems like you might have a bug. Is this cygwin? 
If not try it.

James
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Re: Using XML w/ Python...

[James]

ActivePython is same as Standard Python distribution but with a few
extras.

"As far as it should work since their both transparent, umm, well its
not."

Why do you think it is not transparent? Did you try installing it on
both?
I have ActivePython 2.4 here and it loads amara fine.

"
Traceback (most recent call last):
  File "", line 1, in ?
ImportError: No module named amara
"

That means you did not manage to install it properly. Are you new to
installing Python modules from command line? If you need more hand
holding, try the Python IRC channel on freenode. The responses will be
more in real time. You probably need that since you seem to have more
than one thing to learn about.

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Re: Random Number Generation?

[Mike Meyer]

[EMAIL PROTECTED] (Bengt Richter) writes:
> Theoretically, the chances of getting an integer from a uniformly
> random sample from an interval of real numbers is practically zero,
> and even allowing for IEEE 754 double representation,

Well, if we're going to be picky, the chances of getting a number with
an IEEE 754 representation from a uniformly random sample from an
interval of real numbers is practically zero. Of course, this is true
for *any* finite subset of the reals (such as the set of numbers that
have names that can be pronounced in the average human lifespan), and
probably an infinite number of infinite subsets as well.

But I tend to pick irrationals when asked to "pick a number between 1
and 10."

> So what do you mean by "integer"?
> And what by "decimals"?

I think we should start by finding out what he means by "number",
which is apparently a superset of both what he means by "integer" and
"decimals".

  http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.
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Re: OO in Python? ^^

[Bengt Richter]

On Mon, 12 Dec 2005 01:12:26 +, Tom Anderson <[EMAIL PROTECTED]> wrote:
>tom
>
>-- 
>ø¤º°`°º¤øø¤º°`°º¤øø¤º°`°º¤øø¤º°`°º¤ø
>---910079544-1780890058-1134349946=:30272--

[OT} (just taking liberties with your sig ;-)
,<@><
  °º¤øø¤º°`°º¤øø¤º°P`°º¤ø,,y,,ø¤º°t`°º¤ø,,h,,ø¤º°o`°º¤ø,,n,,ø¤º°


Regards,
Bengt Richter
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Re: OO in Python? ^^

[Alex Martelli]

Tom Anderson <[EMAIL PROTECTED]> wrote:
   ...
> Haskell is strongly and statically typed - very strongly and very 
> statically!

Sure.


> However, what it's not is manifestly typed - you don't have to put the
> types in yourself; rather, the compiler works it out. For example, if i
> wrote code like this (using python syntax):
> 
> def f(x):
>   return 1 + x
> 
> The compiler would think "well, he takes some value x, and he adds it to 1
> and 1 is an integer, and the only thing you can add to an integer is 
> another integer, so x must be an integer; he returns whatever 1 + x works
> out to, and 1 and x are both integers, and adding two integers makes an
> integer, so the return type must be integer", and concludes that you meant

hmmm, not exactly -- Haskell's not QUITE as strongly/rigidly typed as
this... you may have in mind CAML, which AFAIK in all of its variations
(O'CAML being the best-known one) *does* constrain + so that "the only
thing you can add to an integer is another integer".  In Haskell, + can
sum any two instances of types which meet typeclass Num -- including at
least floats, as well as integers (you can add more types to a typeclass
by writing the required functions for them, too).  Therefore (after
loading in ghci a file with
f x = x + 1
), we can verify...:

*Main> :type f
f :: (Num a) => a -> a


A very minor point, but since the need to use +. and the resulting lack
of polymorphism are part of what keeps me away from O'CAML and makes me
stick to Haskell, I still wanted to make it;-).


Alex
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Re: Using XML w/ Python...

[Jay]

Ummm, my error conditions.
PythonWin 2.3.5 (#62, Feb  9 2005, 16:17:08) [MSC v.1200 32 bit
(Intel)] on win32.
Portions Copyright 1994-2004 Mark Hammond ([EMAIL PROTECTED]) -
see 'Help/About PythonWin' for further copyright information.

>>> import amara
Traceback (most recent call last):
  File "", line 1, in ?
ImportError: No module named amara

Pretty straight forward
As far as it should work since their both transparent, umm, well its
not.
But what would be a help would be if u knew the install dir for
ActivePython so maybe i can install amara stand alone into the
ActivePython installation dir. ?? Maybe

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Re: TypeError: no arguments expected

[Alex Martelli]

shawn a <[EMAIL PROTECTED]> wrote:
   ...
> Ive looked around for this exeption but nothing ive read has help in
> this situation.
> Any of your thoughts are greatly apprectiated. THANK!!

Add "self" as the first or only parameters to all methods.

Re-read the tutorial -- you're missing very crucial parts of it.


Alex
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Re: TypeError: no arguments expected

[Roy Smith]

In article <[EMAIL PROTECTED]>,
 [EMAIL PROTECTED] wrote:

> put "self" as the first argument in all instance methods of that class.
> "self" is just a convention, referring to the object instance, not a
> language feature as in other language like javascript. You can call it
> "me" too if you prefer.

But please don't call it "me".  The convention to call the first argument 
"self" is so universal, calling it anything else is just going to make your 
code more difficult for anybody else to read.
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Re: TypeError: no arguments expected

[bonono]

shawn a wrote:
> I havet these 2 files in the same dir. This is code im writing to learn 
> pythong
> mkoneurl.py:
> #! /usr/bin/env python
>
> import make_ou_class
>
> run = make_ou_class.makeoneurl()
>
> 
> make_ou_class.py:
>
> class makeoneurl:
> def __init__():
> self.commandline()
>
> def commandline():
> com = raw_input(":")
> #Parse out any params and aguements - reg expressions
> #params[] array to hold paramters
> params = 0
> if com == "ou":
> self.ou(params)
> else:
> print com + " unknown command."
>
> def ou(params):
> print "hello world"
> self.commandline()
>
> ===
> when i run the script like this: python mkoneurl.py
> I get this error:
>
> Traceback (innermost last):
>   File "mkoneurl.py", line 5, in ?
> run = make_ou_class.makeoneurl()
> TypeError: no arguments expected
>
> Ive looked around for this exeption but nothing ive read has help in
> this situation.
> Any of your thoughts are greatly apprectiated. THANK!!
>
put "self" as the first argument in all instance methods of that class.
"self" is just a convention, referring to the object instance, not a
language feature as in other language like javascript. You can call it
"me" too if you prefer.

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TypeError: no arguments expected

[shawn a]

I havet these 2 files in the same dir. This is code im writing to learn pythong
mkoneurl.py:
#! /usr/bin/env python

import make_ou_class

run = make_ou_class.makeoneurl()


make_ou_class.py:

class makeoneurl:
def __init__():
self.commandline()

def commandline():
com = raw_input(":")
#Parse out any params and aguements - reg expressions
#params[] array to hold paramters
params = 0
if com == "ou":
self.ou(params)
else:
print com + " unknown command."

def ou(params):
print "hello world"
self.commandline()

===
when i run the script like this: python mkoneurl.py
I get this error:

Traceback (innermost last):
  File "mkoneurl.py", line 5, in ?
run = make_ou_class.makeoneurl()
TypeError: no arguments expected

Ive looked around for this exeption but nothing ive read has help in
this situation.
Any of your thoughts are greatly apprectiated. THANK!!

--shawn
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Re: Random Number Generation?

[Bengt Richter]

On Sun, 11 Dec 2005 09:46:33 -0800 (PST), Dimos <[EMAIL PROTECTED]> wrote:

>Hello All,
>
>I need some help with random number generation. What I
>need exactly is:
>
>To create a few thousand numbers, decimal and
>integers, between 5 and 90, 
>and then to export them as a single column at a
>spreadsheet.
>
>I am newbie, I was not able to create decimals with
>the random modules of 
>Python 2.3.
>
Others have mentioned random.random and, better for your use case,
random.uniform, but I'm not sure what you mean by "decimal and integers".

Theoretically, the chances of getting an integer from a uniformly random
sample from an interval of real numbers is practically zero, and even
allowing for IEEE 754 double representation, the realtive population of
integers vs non-integers is pretty low. So what do you mean by "integer"?
And what by "decimals"?

If you just want an artificial sprinkling of exact integer values to
happen some percentage of the time, you could do something like

 >>> from random import uniform, random
 >>> def urnmix(nnum=20, lo=5, hi=90, percentint=25):
 ... percentint /= 100.
 ... for _ in xrange(nnum):
 ... u = uniform(5, 90)
 ... if random()>> for u in urnmix(12): print '%6.3f'%u,
 ...
  9.000 38.173 59.829 37.090 80.504 34.000 69.989 26.000 72.502 64.000 55.000  
9.043
 >>> for u in urnmix(12): print '%6.3f'%u,
 ...
 10.000 67.687 70.323 66.672 17.150 68.447 84.406  6.997 82.444  8.001 82.946 
34.849
 >>> for u in urnmix(12): print '%6.3f'%u,
 ...
 70.000 64.000 36.537 75.270 67.000 70.873 28.446 18.483 75.086 41.703 82.885 
30.558
 >>> for u in urnmix(12): print '%6.3f'%u,
 ...
 75.000 78.313 76.873 48.364 12.000 40.000 36.962 27.704  8.814 44.078 61.000 
35.654
 >>>

Hm, let's check the percentages

 >>> [u==int(u) for u in urnmix(12)]
 [True, False, False, False, False, True, True, True, True, False, True, False]
 >>> [u==int(u) for u in urnmix(12)].count(True)
 2
 >>> [u==int(u) for u in urnmix(12)].count(True)
 4
 >>> [u==int(u) for u in urnmix(1)].count(True)
 2471
 >>> [u==int(u) for u in urnmix(1)].count(True)
 2539
 
Seems to work ...

 >>> [u==int(u) for u in urnmix(1, 5, 90, 5)].count(True)
 497
 >>> [u==int(u) for u in urnmix(1, 5, 90, 1)].count(True)
 111
 >>> [u==int(u) for u in urnmix(1, 5, 90, 1)].count(True)
 87
 >>> [u==int(u) for u in urnmix(1, 5, 90, 1)].count(True)
 113

After all this playing, what was it you actually wanted? ;-)

Regards,
Bengt Richter
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Re: wxpython book

[Bugs]

Here's a more direct link:
http://www.manning.com/books/rappin
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Re: Another newbie question

[Mike Meyer]

[EMAIL PROTECTED] (Alex Martelli) writes:
> Mike Meyer <[EMAIL PROTECTED]> wrote:
>...
>> Except you haven't shown that the API was badly designed. You can't
>> show that it's badly designed, because you don't know the requirements
>> that the API is meeting.
> I can show that an API is badly designed *whatever requirements it might
> be intended for* if it's self-contradictory: containing a way to CHANGE
> an attribute to some different value, but not containing any way to SET
> THAT ATTRIBUTE TO THE RIGHT VALUE from the beginning, is inherently an
> indicator of bad design, because it needlessly imposes more work on the
> API's user and forces objects to pass through a transient state in which
> their attributes are WRONG, or MEANINGLESS.

Nope. If the requirements are that all objects start in the same
meaningful state, then you simply create them in that state. There's
no need to provide a facility to to set the initial state, and they
never go through a meaningless state either.

>> >  And I showed that, in the GENERAL case, since attributes
>> > worth being made assignable are obviously also worth being made settable
>> > in a constructor of some kind,
>> But we're not dealing with a general case, we're dealing with a
>> specific case. Just because you can't think of cases where an
>> attribute being settable doesn't mean it needs to be settable in a
>> constructor doesn't mean they don't exist.
> The burden of the proof is on you, of course: show a design situation
> where it's RIGHT to force API users to do extra work and lead objects
> through states they're *NOT* meant to be in, because there is no way to
> build the object correctly from the start, but rather the object must be
> built in a wrong state and then later coerce it to the state you knew
> was right since the beginning.

You're doing it again - I never claimed that there was any such API
requirement. You've reached this conclusion on your own, by adding
requirements to the design that I never discussed. If you want to
someone to proof this straw man, you'll have to do it yourself.

>> > So, I claim I have totally disproven your claims about difficulty
>> > ("extra work", as you're trying to weaselword your way out, might be
>> > writing one or two trivial lines of code, but that's not DIFFICULT, and
>> > the claim you originally made was about DIFFICULTY, not tiny amounts of
>> > trivially easy "extra work" -- as I already mentioned, obviously ANY
>> > method you add is "extra work" for you compared to not adding it, but
>> > the interesting question is whether that entails any DIFFICULTY).
>> Actually, the original claim was "more difficult". You've done your
>> usual trick of reaching an invalid conclusion from what someone said,
>> then acting as if that's what they said. Congratulations, you've
>> successfully beaten up the straw man you created.
> Right: I claim, on the other hand, that YOU are weaselwording, by trying
> to claim that any class with one extra method is thereby "MORE
> DIFFICULT" to write -- equating having to write one or two lines of
> trivial code with "MORE DIFFICULT" would make the "more difficult"
> totally bereft of any useful meaning in whatever context.

As I already explained, the entire change was trivial, so any extra
work is of course trivial. This extra work is exactly what I meant
when I said "more difficult". You want to play semantic games, and
argue that one trivial change can't be "more difficult" than another,
feel free to do so. But do realize that you're disproving your
strawmen, not my statement.

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Re: OO in Python? ^^

[Alex Martelli]

Mike Meyer <[EMAIL PROTECTED]> wrote:

> Steven D'Aprano <[EMAIL PROTECTED]> writes:
> > Of course, the IT world is full of people writing code and not testing
> > it, or at least not testing it correctly. That's why there are frequent
> > updates or upgrades to software that break features that worked in the
> > older version. That would be impossible in a test-driven methodology, at
> > least impossible to do by accident.
> 
> That sentence is only true if your tests are bug-free. If not, it's
> possible to make a change that introduces a bug that passes testing
> because of a bug in the tests. Since tests are code, they're never
> bug-free. I will agree that the frequency of upgrades/updates breaking
> things means testing isn't being done properly.

Yours is a good point: let's be careful not to oversell or overhype TDD,
which (while great) is not a silver bullet.  Specifically, TDD is prone
to a "common-mode failure" between tests and code: misunderstanding of
the specs (generally underspecified specs); since the writer of the test
and of the code is the same person, if that person has such a
misunderstanding it will be reflected equally in both code and test.

Which is (part of) why code developed by TDD, while more robust against
many failure modes than code developed more traditionally, STILL needs
code inspection (or pair programming), integration tests, system tests,
and customer acceptance tests (not to mention regression tests, once
bugs are caught and fixed;-), just as much as code developed otherwise.


Alex
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Re: Using XML w/ Python...

[Jay]

Ok, im convinced to that i need to get Amara, I just installed 4Suite
and now installed Amara. Still doesnt work because like i said before,
i use ActivePython from

http://www.activestate.com/Products/ActivePython/

And the requirements for Amara is Python 2.4 so Thats where we have
a problem, i need Amara for ActivePython. And i would like to keep
working on ActivePython w/o downloading Python 2.4.

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newbie question about python and Tkinter

[newbie]

Hello,

I am a newbie and have a few questions about Python and Tkinter.

I would like to create the following layout in Python:

***
* *
* *
* *
* frame 1 *
* *
* *
* *
* *
***
*   * *
*   * *
*   * *
*frame 3*  frame 2*
*   * *
*   * *
*   * *
***

I try to accomplish this with the following code:

from Tkinter import *

root = Tk()
root.title("Critter World")

MyFrame0 = Frame(root, background="brown")  # One Parent Frame to rule
them all

MyFrame1  = Frame(MyFrame0, background = "yellow")
MyFrame2  = Frame(MyFrame0, background = "red")
MyFrame3  = Frame(MyFrame0, background = "green")

b1 = Label(MyFrame1, text="World", background="yellow").pack(fill=BOTH,
expand=YES)
b2 = Label(MyFrame2, text="Info", background="red").pack(fill=BOTH,
expand=YES)
b3 = Label(MyFrame3, text="Control",
background="green").pack(fill=BOTH, expand=YES)

MyFrame1.pack(side=TOP, fill=BOTH, expand=YES)
MyFrame2.pack(side=RIGHT, fill=BOTH, expand=YES)
MyFrame3.pack(side=LEFT, fill=BOTH, expand=YES)

MyFrame0.pack(fill=BOTH, expand=YES)
raw_input()
print "\n"*23

When the window resizes, it is only Frame1 that is resized.




However, if I run the following:



from Tkinter import *

root = Tk()
root.title("Critter World")

MyFrame0 = Frame(root, background="brown")  # One Parent Frame to rule
them all

MyFrame1  = Frame(MyFrame0, background="yellow")
MyFrame2  = Frame(MyFrame0, background = "red")
MyFrame3  = Frame(MyFrame0, background = "green")
MyFrame4  = Frame(MyFrame0, background = "black")


b1 = Label(MyFrame1, text="World", background="yellow").pack(fill=BOTH,
expand=YES)
b2 = Label(MyFrame2, text="Info", background="red").pack(fill=BOTH,
expand=YES)
b3 = Label(MyFrame3, text="Control",
background="green").pack(fill=BOTH, expand=YES)
b4 = Label(MyFrame3, text="NEXT 1", background="green").pack(fill=BOTH,
expand=YES)


MyFrame1.pack(side=TOP, fill=BOTH, expand=YES)
MyFrame2.pack(side=RIGHT, fill=BOTH, expand=YES)
MyFrame3.pack(side=LEFT, fill=BOTH, expand=YES)
MyFrame4.pack(side=BOTTOM, fill=BOTH, expand=YES)

MyFrame0.pack(fill=BOTH, expand=YES)
raw_input()
print "\n"*23

I get:

***
* *
* *
* *
* frame 1 *
* *
* *
* *
* *
* *
* *
* *
* *
* *
***
*   * *
*   * *
*frame 3* *
*   * *
*   * *
*  frame 2*
*   * *
*   * *
*frame 4* *
*   * *
*   * *
***

and when I resize, all frames are resized?

Any idea why?


A related question:

Why does frame 4 not span the bottom ?

i.e.

***
* *
* *
* *
* frame 1 *
* *
* *
* *
* *
* *
* *
* *
* *
* *
***
*   * *
*   * *
*frame 2*frame 3  *
*   * *
*   * *
*

Re: OO in Python? ^^

[Tom Anderson]

On Mon, 12 Dec 2005, Steven D'Aprano wrote:


On Sun, 11 Dec 2005 05:48:00 -0800, bonono wrote:

And I don't think Haskell make the programmer do a lot of work(just 
because of its static type checking at compile time).


I could be wrong, but I think Haskell is *strongly* typed (just like
Python), not *statically* typed.


Haskell is strongly and statically typed - very strongly and very 
statically!


However, what it's not is manifestly typed - you don't have to put the 
types in yourself; rather, the compiler works it out. For example, if i 
wrote code like this (using python syntax):


def f(x):
return 1 + x

The compiler would think "well, he takes some value x, and he adds it to 1 
and 1 is an integer, and the only thing you can add to an integer is 
another integer, so x must be an integer; he returns whatever 1 + x works 
out to, and 1 and x are both integers, and adding two integers makes an 
integer, so the return type must be integer", and concludes that you meant 
(using Guido's notation):


def f(x: int) -> int:
return 1 + x

Note that this still buys you type safety:

def g(a, b):
c = "{" + a + "}"
d = 1 + b
return c + d

The compiler works out that c must be a string and d must be an int, then, 
when it gets to the last line, finds an expression that must be wrong, and 
refuses to accept the code.


This sounds like it wouldn't work for complex code, but somehow, it does. 
And somehow, it works for:


def f(x):
return x + 1

Too. I think this is due to the lack of polymorphic operator overloading.

A key thing is that Haskell supports, and makes enormous use of, a 
powerful system of generic types; with:


def h(a):
return a + a

There's no way to infer concrete types for h or a, so Haskell gets 
generic; it says "okay, so i don't know what type a is, but it's got to be 
something, so let's call it alpha; we're adding two alphas, and one thing 
i know about adding is that adding two things of some type makes a new 
thing of that type, so the type of some-alpha + some-alpha is alpha, so 
this function returns an alpha". ISTR that alpha gets written 'a, so this 
function is:


def h(a: 'a) -> 'a:
return a + a

Although that syntax might be from ML. This extends to more complex 
cases, like:


def i(a, b):
return [a, b]

In Haskell, you can only make lists of a homogenous type, so the compiler 
deduces that, although it doesn't know what type a and b are, they must be 
the same type, and the return value is a list of that type:


def i(a: 'a, b: 'a) -> ['a]:
return [a, b]

And so on. I don't know Haskell, but i've had long conversations with a 
friend who does, which is where i've got this from. IANACS, and this could 
all be entirely wrong!


At least the "What Is Haskell?" page at haskell.org describes the 
language as strongly typed, non-strict, and allowing polymorphic typing.


When applied to functional languages, 'strict' (or 'eager'), ie that 
expressions are evaluated as soon as they are formed; 'non-strict' (or 
'lazy') means that expressions can hang around as expressions for a while, 
or even not be evaluated all in one go. Laziness is really a property of 
the implementation, not the the language - in an idealised pure functional 
language, i believe that a program can't actually tell whether the 
implementation is eager or lazy. However, it matters in practice, since a 
lazy language can do things like manipulate infinite lists.


tom

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Re: Another newbie question

[Alex Martelli]

Mike Meyer <[EMAIL PROTECTED]> wrote:
   ...
> Except you haven't shown that the API was badly designed. You can't
> show that it's badly designed, because you don't know the requirements
> that the API is meeting.

I can show that an API is badly designed *whatever requirements it might
be intended for* if it's self-contradictory: containing a way to CHANGE
an attribute to some different value, but not containing any way to SET
THAT ATTRIBUTE TO THE RIGHT VALUE from the beginning, is inherently an
indicator of bad design, because it needlessly imposes more work on the
API's user and forces objects to pass through a transient state in which
their attributes are WRONG, or MEANINGLESS.


> >  And I showed that, in the GENERAL case, since attributes
> > worth being made assignable are obviously also worth being made settable
> > in a constructor of some kind,
> 
> But we're not dealing with a general case, we're dealing with a
> specific case. Just because you can't think of cases where an
> attribute being settable doesn't mean it needs to be settable in a
> constructor doesn't mean they don't exist.

The burden of the proof is on you, of course: show a design situation
where it's RIGHT to force API users to do extra work and lead objects
through states they're *NOT* meant to be in, because there is no way to
build the object correctly from the start, but rather the object must be
built in a wrong state and then later coerce it to the state you knew
was right since the beginning.

There may be languages which are so feeble as to force such behavior
(e.g., languages where every new instance has every attribute forced to
null even where it makes no sense for a certain attribute to ever be
null) but that applies to neither Eiffel nor Python, and all it shows is
that some languages are seriously lacking in the tools to allow proper
designs to be implemented, not that "all objects must always be
generated in the WRONG state" can ever be the RIGHT design.


> > So, I claim I have totally disproven your claims about difficulty
> > ("extra work", as you're trying to weaselword your way out, might be
> > writing one or two trivial lines of code, but that's not DIFFICULT, and
> > the claim you originally made was about DIFFICULTY, not tiny amounts of
> > trivially easy "extra work" -- as I already mentioned, obviously ANY
> > method you add is "extra work" for you compared to not adding it, but
> > the interesting question is whether that entails any DIFFICULTY).
> 
> Actually, the original claim was "more difficult". You've done your
> usual trick of reaching an invalid conclusion from what someone said,
> then acting as if that's what they said. Congratulations, you've
> successfully beaten up the straw man you created.

Right: I claim, on the other hand, that YOU are weaselwording, by trying
to claim that any class with one extra method is thereby "MORE
DIFFICULT" to write -- equating having to write one or two lines of
trivial code with "MORE DIFFICULT" would make the "more difficult"
totally bereft of any useful meaning in whatever context.

I'm currently in an interesting job role, known as "uber technical
lead", which is meant to be a sort of a cross between technical manager
and ubergeek-guru.  Fortunately, my reports are all people of technical
excellence as well as personal integrity, so, should I ever ask one of
them to explain why he or she did X and not Y, I fully trust they won't
try to explain that "doing Y would have been more difficult" when the
reality is that it would have involved a line of two of trivial code...
if they did, I can assure you that the consequences might be
interesting.  (Good think I can and do trust them to say, should such a
situation ever arise, "DUH! -- I just didn't think of it!", and go fix
their code forthwith... just as they've often heard ME say,
apologetically, in the much more frequent situations where my objections
to some design were misconceived... so, my modest management abilities
will not be put to such a difficult test in the foreseeable future;-).


Alex
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Re: Using XML w/ Python...

[uche . ogbuji]

"""
 Ok, i am now understanding some of parseing and how to use it and
nodes, things like that. But say i wanted to take the title of
http://www.digg.com/rss/index.xml

and XMLTramp seemed the most simple to understand.

would the path be something like this?

import xmltramp
rssDigg = xmltramp.load("http://www.digg.com/rss/index.xml";)
print note.rss.channel.item.title

I think thats wat im having the most confusion on now, is how to direct
to the path that i want...
"""

I suggest you read at least the front page information for the tools
you are using.  It's quite clear from the xmltramp Web site (
http://www.aaronsw.com/2002/xmltramp/ ) that you want tomething like
(untested: the least homework you can do is to refine the example
yourself):

print rssDigg[rss.channel][item][title]

BTW, in Amara, the API is pretty much exactly what you guessed:

>>> import amara
>>> rssDigg = amara.parse("http://www.digg.com/rss/index.xml";)
>>> print rssDigg.rss.channel.item.title
Video: Conan O'Brien iPod Ad Parody


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http://uche.ogbuji.nethttp://fourthought.com
http://copia.ogbuji.net   http://4Suite.org
Articles: http://uche.ogbuji.net/tech/publications/

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Re: ANN: Dao Language v.0.9.6-beta is release!

[Tom Anderson]

On Sun, 11 Dec 2005, Steven D'Aprano wrote:


On Sat, 10 Dec 2005 16:34:13 +, Tom Anderson wrote:


On Sat, 10 Dec 2005, Sybren Stuvel wrote:


Zeljko Vrba enlightened us with:

Find me an editor which has folds like in VIM, regexp search/replace 
within two keystrokes (ESC,:), marks to easily navigate text in 2 
keystrokes (mx, 'x), can handle indentation-level matching as well as 
VIM can handle {}()[], etc.  And, unlike emacs, respects all (not 
just some) settings that are put in its config file. Something that 
works satisfactorily out-of-the box without having to learn a new 
programming language/platform (like emacs).


Found it! VIM!


ED IS THE STANDARD TEXT EDITOR.


Huh! *Real* men edit their text files by changing bits on the hard disk 
by hand with a magnetized needle.


Hard disk? HARD DISK?

Hard disks are for losers who can't write tight code. *Real* mean keep 
everything in core. Unless it's something performance-critical, in which 
case they fit it in the cache.


tom

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Pythonic XML library with XPath support for Jython?

[James]

A couple of years ago there wasn't one and the recommendation was to
simply use Java libs. Have things changed since?

I see ElementTree promises one in the future but are there any out now?

Thanks.

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Re: instance + classmethod question

[Mike Meyer]

Laszlo Zsolt Nagy <[EMAIL PROTECTED]> writes:
> Is it possible to tell, which instance was used to call the
> classmethod that is currently running?

Ok, I read through what got to my nntp server, and I'm still
completely confused.

A class method isn't necessarilry called by an instance. That's why
it's a class method. What should happen in that case?

You provided an example where you passed self as an optional
argument. If it's going to have self, shouldn't it be an instance
method?

I think I agree with Steven - you should use two methods. You deal
with the issue of duplicated code by pulling the code that would be
duplicated out into private methods. This would be a straightforward
refactoring problem.

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Re: Another newbie question

[Mike Meyer]

[EMAIL PROTECTED] (Alex Martelli) writes:
> Mike Meyer <[EMAIL PROTECTED]> wrote:
>...
>> It's not my cherished example - it actually came from someone
> You picked it to (try and fail to) show that there is DIFFICULTY, which
> I showed there isn't.

No, you showed you could change the example so there is no extra
difficulty.

>> else. That you can change the requirements so that there is no extra
>> work is immaterial - all you've done is shown that there are examples
>> where that don't require extra work. I never said that such examples
>> didn't exist. All you've shown - in both the single concrete example
>> and in a generalized case - is that any requirement can be changed so
>> that it doesn't require any extra work. This doesn't change the fact
>> that such cases exist, which is all that I claimed was the case.
> Untrue: you claimed that the specific API (allowing attribute-setting)
> "makes changing the object more difficult", not the obvious fact that
> "there exist APIs so badly designed that they make changing more
> difficult".

Except you haven't shown that the API was badly designed. You can't
show that it's badly designed, because you don't know the requirements
that the API is meeting.

>  And I showed that, in the GENERAL case, since attributes
> worth being made assignable are obviously also worth being made settable
> in a constructor of some kind,

But we're not dealing with a general case, we're dealing with a
specific case. Just because you can't think of cases where an
attribute being settable doesn't mean it needs to be settable in a
constructor doesn't mean they don't exist.

> So, I claim I have totally disproven your claims about difficulty
> ("extra work", as you're trying to weaselword your way out, might be
> writing one or two trivial lines of code, but that's not DIFFICULT, and
> the claim you originally made was about DIFFICULTY, not tiny amounts of
> trivially easy "extra work" -- as I already mentioned, obviously ANY
> method you add is "extra work" for you compared to not adding it, but
> the interesting question is whether that entails any DIFFICULTY).

Actually, the original claim was "more difficult". You've done your
usual trick of reaching an invalid conclusion from what someone said,
then acting as if that's what they said. Congratulations, you've
successfully beaten up the straw man you created.

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Re: Displaying error message in a try except?

[Tim Williams (gmail)]

On 11/12/05, Fredrik Lundh <[EMAIL PROTECTED]> wrote:
[EMAIL PROTECTED] wrote:> Fairly new to python.  In a try except how do you display the true> (raw) error message so it can be displayed back to the user?
assuming that "true" means "the message you would get if you hadn'tused a try/except", the traceback module is what you want:import tracebacktry:raise SyntaxError("example")
except:traceback.print_exc()printsTraceback (innermost last):  File "module.py", line 4, in ?SyntaxError: examplemore here:
http://effbot.org/librarybook/traceback.htmyou can also inspect the exception status via the sys.exc_info() call.e.g.import systry:1/0except:print "%s: %s" % 
sys.exc_info()[:2]printsexceptions.ZeroDivisionError: integer division or modulo by zero
Or you can combine both,  I find these 3 lines after the except
most helpful during developement stages and liberally splatter them
around prototype code 

>>> import sys, traceback
>>> try:
...     1/0
... except:
... print sys.exc_info()[0]
... print sys.exc_info()[1]
... print "LINE=",traceback.tb_lineno(sys.exc_info()[2])
... 
exceptions.ZeroDivisionError
integer division or modulo by zero
LINE= 2>>>
HTH :)


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Re: ANN: Dao Language v.0.9.6-beta is release!

[Mike Meyer]

Zeljko Vrba <[EMAIL PROTECTED]> writes:
> An obvious defficieny of the current way we write code now is its inherent
> tree-structure resulting from {}, indentation, begin/end markers or whatnot.
> But the flow of code is often not a tree but a cycle.. Yet we are always
> dealing with a tree-like representation of the code on screen.

Except the the representation on the screen isn't tree-like, it's a
two-dimenional array of characters. You can, of course, interpret that
as tree-like structure. But you can also interpret it as generalized
graph, cycles included.

> I have no problem accepting that I'm in a minority. I have a problem with
> offensive people using my example arguments to riducule me.. While they
> even don't want to open their minds and accept that there might be a
> remote possibility that indentation is not ideal.

What surprises me is that people come in assuming that there must be
one language for everything and everyone, and that Python should
strive to be that language. Such a language doesn't exist, so there is
no such thing as 'ideal'. A language that might be optimal for one
person and task won't necessarily be optimal if you change either the
person or the task. Yet some people intent on forcing one language to
be ideal propose changing it to suit them, and then act surprised when
people who've tried their proposed changes in other languages and
found them lacking reject them.


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Re: OO in Python? ^^

[Mike Meyer]

Bruno Desthuilliers <[EMAIL PROTECTED]> writes:
>>  ^^ There is no functionality to check if a subclass correctly
>> implements an inherited interface
>
> I don't know of any language that provide such a thing. At least for
> my definition of "correctly".

Well, since your definition of "correclty" is uknown, I won't use
it. I will point out that the stated goal is impossible for some
reasonable definitions of "correctly".

My definition of "correctly" is "meets the published contracts for the
methods."  Languages with good support for design by contract will
insure that subclasses either correctly implement the published
contracts, or raise an exception when they fail to do so. They do that
by checking the contracts for the super classes in an appropriate
logical relationship, and raising an exception if the contract isn't
met.

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Re: OO in Python? ^^

[Mike Meyer]

Steven D'Aprano <[EMAIL PROTECTED]> writes:
> Of course, the IT world is full of people writing code and not testing
> it, or at least not testing it correctly. That's why there are frequent
> updates or upgrades to software that break features that worked in the
> older version. That would be impossible in a test-driven methodology, at
> least impossible to do by accident.

That sentence is only true if your tests are bug-free. If not, it's
possible to make a change that introduces a bug that passes testing
because of a bug in the tests. Since tests are code, they're never
bug-free. I will agree that the frequency of upgrades/updates breaking
things means testing isn't being done properly.

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Re: Using XML w/ Python...

[Jay]

some great suggestions.
Ok, i am now understanding some of parseing and how to use it and
nodes, things like that. But say i wanted to take the title of
http://www.digg.com/rss/index.xml

and XMLTramp seemed the most simple to understand.

would the path be something like this?

import xmltramp
rssDigg = xmltramp.load("http://www.digg.com/rss/index.xml";)
print note.rss.channel.item.title


I think thats wat im having the most confusion on now, is how to direct
to the path that i want... 
Suggestions?

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Re: Displaying error message in a try except?

[Xavier Morel]

Fredrik Lundh wrote:
  > assuming that "true" means "the message you would get if you hadn't
 > used a try/except", the traceback module is what you want:
 >
 > you can also inspect the exception status via the sys.exc_info() call.
 > e.g.
 >
There is also the third way of catching an exception explicitly and 
printing it's arguments and class (doesn't give exactly the same 
information, but gives relevant informations nonetheless)

--
 >>> try:
1/0
except ZeroDivisionError, e:
print e
print e.args
print repr(e)


integer division or modulo by zero
('integer division or modulo by zero',)

--

(catching Exception instead of ZeroDivisionError would yield the same 
result, but would also act as an Exception trap that voids any exception 
raised)
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Re: XML and namespaces

[Paul Boddie]

Alan Kennedy wrote:
> Serialisation and namespace normalisation are both in the realm of DOM
> Level 3, whereas minidom is only L2 compliant. Automagically introducing
> L3 semantics into the L2 implementation is the wrong thing to do.

I think I'll have to either add some configuration support, in order to
let the user specify which standards they have in mind, or to
deny/assert support for one or another of the standards. It's
interesting that minidom plus PrettyPrint seems to generate the xmlns
attributes in the serialisation, though; should that be reported as a
bug?

As for the toxml method in minidom, the subject did seem to be briefly
discussed on the XML-SIG mailing list earlier in the year:

http://mail.python.org/pipermail/xml-sig/2005-July/011157.html

> its-not-about-namespaces-its-about-automagic-ly'yrs

Well, with the automagic, all DOM users get the once in a lifetime
chance to exchange those lead boots for concrete ones. I'm sure there
are all sorts of interesting reasons for assigning namespaces to nodes,
serialising the document, and then not getting all the document
information back when parsing it, but I'd rather be spared all the
"amusement" behind all those reasons and just have life made easier for
just about everyone concerned. I think the closing remarks in the
following message say it pretty well:

http://mail-archives.apache.org/mod_mbox/xml-security-dev/200409.mbox/<1095071819.17967.44.camel%40amida>

And there are some interesting comments on this archived page, too:

http://web.archive.org/web/20010211173643/http://xmlbastard.editthispage.com/discuss/msgReader$6

Anyway, thank you for your helpful commentary on this matter!

Paul

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Re: OO in Python? ^^

[Mike Meyer]

"Paul Boddie" <[EMAIL PROTECTED]> writes:
> One classic example of a
> weakly-typed language is BCPL, apparently, but hardly anyone has any
> familiarity with it any more.

Actually, BCPL is what Stevenn D'Aprano called "untyped". Except his
definition is suitable for after everyone followed IBM's footsteps in
building general-purpose byte-addressable machines.

In BCPL, everything is a word. Given a word, you can dereference it,
add it to another word (as either a floating point value or an integer
value), or call it as a function.

A classic example of a weakly-typed language would be a grandchild of
BCPL, v6 C. Since then, C has gotten steadily more strongly typed. A
standard complaint as people tried to move code from a v6 C compiler
(even the photo7 compiler) to the v7 compiler was "What do you mean I
can't ". Of course, hardly anyone has familiarity with that any
more, either.

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Re: Displaying error message in a try except?

[Fredrik Lundh]

[EMAIL PROTECTED] wrote:

> Fairly new to python.  In a try except how do you display the true
> (raw) error message so it can be displayed back to the user?

assuming that "true" means "the message you would get if you hadn't
used a try/except", the traceback module is what you want:

import traceback

try:
raise SyntaxError("example")
except:
traceback.print_exc()

prints

Traceback (innermost last):
  File "module.py", line 4, in ?
SyntaxError: example

more here:

http://effbot.org/librarybook/traceback.htm

you can also inspect the exception status via the sys.exc_info() call.
e.g.

import sys

try:
1/0
except:
print "%s: %s" % sys.exc_info()[:2]

prints

exceptions.ZeroDivisionError: integer division or modulo by zero

for more info, see the library reference.

hope this helps!





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Re: mail attachment with non-ascii name

[Neil Hodgson]

Bernard Delmée:

> I am using the "email" module to decode incoming messages.
> (with msg = email.message_from_file( msg_file ))
> Sometimes an attachment has its name (as returned by
> msg.walk().part.get_filename()) not in ASCII (e.g.
> '=?iso-8859-1?q?somefile=2ezip?=') How can I turn that into
> simply 'somefile.zip' ? I have looked into email.Utils and
> codecs, but cannot find what should work.

 >>> import email.Header
 >>> x = '=?iso-8859-1?q?somefile=2ezip?='
 >>> email.Header.decode_header(x)
[('somefile.zip', 'iso-8859-1')]

Neil
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Displaying error message in a try except?

[wawork]

Fairly new to python.  In a try except how do you display the true
(raw) error message so it can be displayed back to the user?

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Re: instance + classmethod question

[Steven Bethard]

Laszlo Zsolt Nagy wrote:
> In my methods, most code is about string manipulation and calling other 
> classmethods.
> There are only a few places where I can use an instance, but it is not 
> required.
> I would like to reuse as most code as possible, so I do not want to 
> create two different
> methods. That would result in duplicating code.

I would tend to do this by creating a wrapper method for the instance 
that did the appropriate stuff for the instance, and then called the 
classmethod, e.g.:

class C(object):
...
@classmethod
def do_stuff(cls, *args):
...
def do_instance_stuff(self, *args):
# instance stuff
...
self.do_stuff(*args)
# more instance stuff
...

But it does require some factoring of the classmethod so that it makes 
sense to call it in this manner.

STeVe
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Re: Creating referenceable objects from XML

[Alan Kennedy]

[Michael Williams]
> I need it to somehow convert my XML to intuitively referenceable  
> object.  Any ideas?  I could even do it myself if I knew the  mechanism 
> by which python classes do this (create variables on the fly).

You seem to already have a fair idea what kind of model you need, and to 
know that there is a simple way for you to create one. I encourage you 
to progress on this path: it will increase the depth of your understanding.

One mistake I think that some people make about XML is relying on other 
peoples interpretations of the subject, rather than forming their own 
opinions.

The multitude of document models provided by everyone and his mother all 
make assumptions about how the components of the model will be accessed, 
in what order those components will be accessed, how often and when, how 
memory efficient the model is, etc, etc.

To really understand the trade-offs and strengths of all the different 
models, it is a good exercise to build your own object model. It's a 
simple exercise, due to pythons highly dynamic nature. Understanding 
your own model will help you understand what the other models do and do 
not provide. You can then evaluate other off-the-shelf models for your 
specific applications: I always find different XML tools suit different 
situations.

See this post of mine from a couple years back about different ways of 
building your own document/data models.

http://groups.google.com/group/comp.lang.python/msg/e2a4a1c35395ffec

I think the reference to the ActiveState recipe will be of particular 
interest, since you could have a running example very quickly indeed.

See also my tutorial post on extracting document content from a SAX 
stream. I gave the example of a simple stack-based xpath-style 
expression matcher.

http://groups.google.com/group/comp.lang.python/msg/6853bddbb9326948

Also contained in that thread is an illuminating and productive 
discussion between the effbot and myself about how wonderfully simple 
ElementTree makes this, not to mention unbeatably efficient.

this-week-i-ave-been-mostly-using-kid-for-templating-ly'yrs,

-- 
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Re: OO in Python? ^^

[Bruno Desthuilliers]

Matthias Kaeppler a écrit :
(snip)

> I stumbled over this paragraph in "Python is not Java", can anyone 
> elaborate on it:
> 
> "In Java, you have to use getters and setters because using public 
> fields gives you no opportunity to go back and change your mind later to 
> using getters and setters. So in Java, you might as well get the chore 
> out of the way up front. In Python, this is silly, because you can start 
> with a normal attribute and change your mind at any time, without 
> affecting any clients of the class. So, don't write getters and setters."
> 
> Why would I want to use an attribute in Python, where I would use 
> getters and setters in Java? 

Because you don't *need* getters/setters - you already got'em for free 
(more on this latter).

> I know that encapsulation is actually just 
> a hack in Python (common, "hiding" an implementation detail by prefixing 
> it with the classname so you can't access it by its name anymore? Gimme 
> a break...),

You're confusing encapsulation with information hiding. The mechanism 
you're refering to is not meant to 'hide' anything, only to prevent 
accidental shadowing of some attributes. The common idiom for 
"information hiding" in Python is to prefix 'protected' attributes with 
a single underscore. This warns developers using your code that this is 
implementation detail, and that there on their own if they start messing 
with it. And - as incredible as this can be - that's enough.

True, this won't stop stupid programmers from doing stupid thing - but 
no language is 'idiot-proof' anyway (know the old '#define private 
public' C++ trick ?), so why worry ?

> but is that a reason to only write white box classes? ^^

First, what is a 'white box' class ?

public class WhiteBox {
   protected integer foo;
   protected integer bar;

   public integer getFoo() {
 return foo;
   }
   public void setFoo(integer newfoo) {
 foo = newfoo;
   }
   public integer getBar() {
 return bar;
   }
   public void setBar(integer newbar) {
 bar = newbar;
   }
}

Does this really qualify as a 'blackbox' ? Of course not, everything is 
publicly exposed. You could have the exact same result (and much less 
code) with public attributes.

Now what is the reason to write getters and setters ? Answer : so you 
can change the implementation without breaking the API, right ?

Python has something named 'descriptors'. This is a mechanism that is 
used for attribute lookup (notice that everything being an object, 
methods are attributes too). You don't usually need to worry about it 
(you should read about if you really want to understand Python's object 
model), but you can still use it when you need to take control over 
attribute access. One of the simplest application is 'properties' (aka 
computed attributes), and here's an exemple :

class Foo(object):
   def __init__(self, bar, baaz):
 self.bar = bar
 self._baaz = baaz

   def _getbaaz(self):
 return self._baaz

   baaz = Property(fget=_getbaaz)

This is why you don't need explicit getters/setters in Python : they're 
already there ! And you can of course change how they are implemented 
without breaking the interface. Now *this* is encapsulation - and it 
doesn't need much information hiding...
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Re: OO in Python? ^^

[Alex Martelli]

Paul Boddie <[EMAIL PROTECTED]> wrote:
   ...
> offer some kind of solution to that problem. Moreover, Python also lets
> you define double-underscore attribute names which behave give instance
> attributes privacy in all respects, being invisible to users of the
> instances concerned, accessible only within methods belonging to the
> defining class, and safely overloadable in subclasses without incurring
> conflicts.

Unfortunately, that depends on the subclasses' naming:

in module bah.py:
class Foo(object) ...

in module zot.py:
import bah
class Foo(bah.Foo) ...

and alas, the "privacy in all respects" breaks down.  Now, the idea of
"class Foo(bah.Foo)" may look silly with such artificial names, but it
IS somewhat likely to happen in the real world when the classes' names
are meaningful.  I guess pychecker or the like might check for this and
issue a warning if needed... but I do wish we had better ways to prevent
accidental naming conflicts (not that I can easily think of any).


Alex
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Re: Using XML w/ Python...

[uche . ogbuji]

Jay:
"""
K, I have this  XML doc, i dont know much about XML, but what i want
to do is take certain parts of the XML doc, such as  blah
 and take just that and put onto a text doc. Then same thing
doe the  part. Thats about it, i checked out some of the xml
modules but dont understand how to use them. Dont get parsing, so if
you could please explain working with XML and python to me.
"""

Someone already mentioned

http://www.oreillynet.com/pub/wlg/6225

I do want to update that Amara API.  As of recent releases it's as
simple as

import amara
doc = amara.parse("foo.opml")
for url in doc.xpath("//@xmlUrl"):
print url.value

Besides the XPath option, Amara [1] provides Python API options for
unknown elements, such as

node.xml_child_elements
node.xml_attributes

This is all covered with plenty of examples in the manual [2]

[1] http://uche.ogbuji.net/tech/4suite/amara/
[2] http://uche.ogbuji.net/uche.ogbuji.net/tech/4suite/amara/manual-dev

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Articles: http://uche.ogbuji.net/tech/publications/

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Re: Creating referenceable objects from XML

[uche . ogbuji]

Michael Williams wrote:
> Hi All,

> I'm looking for a quality Python XML implementation.  All of the DOM
> and SAX implementations I've come across so far are rather
> convoluted.  Are there any quality implementations that will (after
> parsing the XML) return an object that is accessible by name? Such as
> the following:

> xml = """
> 
>MyBook
>the author
> 
> """

> And after parsing the XML allow me to access it as so:

> book.title

> I need it to somehow convert my XML to intuitively referenceable
> object.  Any ideas?  I could even do it myself if I knew the
> mechanism by which python classes do this (create variables on the fly).

Looks as if MIchael is working with Amara now, but I did want to note
for the record that APIs that allow one to access a node in the
"book.title" fashion are what I call Python data bindings.

Python data bindings I usually point out are:

Amara Bindery: http://www.xml.com/pub/a/2005/01/19/amara.html
Gnosis: http://www.xml.com/pub/a/2003/07/02/py-xml.html
generateDS: http://www.xml.com/pub/a/2003/06/11/py-xml.html

Based on updates to EaseXML in response to my article another entry
might be:

EaseXML: http://www.xml.com/pub/a/2005/07/27/py-xml.html

ElementTree ( http://www.xml.com/pub/a/2003/02/12/py-xml.html ) is a
Python InfoSet rather than a Python data binding.  You access nodes
using generic names related to the node type rather than the node name.
 Whether data bindings or Infosets are your preference is a matter of
taste, but it's a useful distinction to make between the approaches.
It looks as if Gerald Flanagan has constructed a little specialized
binding tool on top of ElementTree, and that's one possible hybrid
approach.

xmltramp ( http://www.aaronsw.com/2002/xmltramp/ ) is another
interesting hybrid.

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Re: OO in Python? ^^

[Bruno Desthuilliers]

Matthias Kaeppler a écrit :
> Hi,
> 
> sorry for my ignorance, but after reading the Python tutorial on 
> python.org, I'm sort of, well surprised about the lack of OOP 
> capabilities in python. 

I beg your pardon ???

> Honestly, I don't even see the point at all of 
> how OO actually works in Python.

> For one, is there any good reason why I should ever inherit from a 
> class?

To specialize it (subtyping), or to add functionnalities (code reuse, 
factoring)

>  ^^ There is no functionality to check if a subclass correctly 
> implements an inherited interface

I don't know of any language that provide such a thing. At least for my 
definition of "correctly".

> and polymorphism seems to be missing 
> in Python as well. 

Could you share your definition of polymorphism ?

> I kind of can't imagine in which circumstances 
> inheritance in Python helps. For example:
> 
> class Base:
>def foo(self): # I'd like to say that children must implement foo
>   pass

class Base(object):
   def foo(self):
 raise NotImplementedError, "please implement foo()"

> class Child(Base):
>pass # works

> Does inheritance in Python boil down to a mere code sharing?

Yes. inheritence is initially made for code sharing (cf Smalltalk). The 
use of inheritence for subtyping comes from restrictions of statically 
typed languages [1]. BTW, you'll notice that the GoF (which is still one 
of the best references about OO) strongly advise to program to 
interfaces, not to implementations. And you'll notice that some patterns 
only exists  as workarounds for the restrictions enforced by statically 
typed languages.


[1] should say : for *a certain class of* statically typed languages. 
There are also languages like OCaml that relies on type inference.

> And how do I formulate polymorphism in Python? 

In OO, polymorphism is the ability for objects of different classes to 
answer the same message. It doesn't imply that these objects should 
inherit from a common base class. Statically typed languages like C++ or 
Java *restrict* polymorphism.


> Example:

(snip)

You don't need any of this.

class Foo:
   def walk(self):
 print "%s walk" % self.__class__.__name__

class Bar:
   def walk(self):
 print "I'm singing in the rain"

def letsgoforawalk(walker):
   walker.walk()

f = Foo()
b = Bar()

letsgoforawalk(f)
letsgoforawalk(b)

Here, the function (BTW, did you know that in Python, functions are 
objects too ?) letsgoforawalk expect an object that has the type 'object 
that understand the message walk()'. Any object of this type will do - 
no need to have a common base class.

> I could as well leave the whole inheritance stuff out and the program 
> would still work (?).

Of course. Why should polymorphism need anything more ?

> Please give me hope that Python is still worth learning :-/

It is, once you've unlearned C++/Java/ADA/whatever.
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mail attachment with non-ascii name

[Bernard Delmée]

I am using the "email" module to decode incoming messages.
(with msg = email.message_from_file( msg_file ))
Sometimes an attachment has its name (as returned by
msg.walk().part.get_filename()) not in ASCII (e.g.
'=?iso-8859-1?q?somefile=2ezip?=') How can I turn that into
simply 'somefile.zip' ? I have looked into email.Utils and
codecs, but cannot find what should work.

TIA,

Bernard.
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Re: XML and namespaces

[Alan Kennedy]

[Paul Boddie]
 > However,
 > wouldn't the correct serialisation of the document be as follows?
 >
 > 
 > 

Yes, the correct way to override a default namespace is an xmlns="" 
attribute.

[Paul Boddie]
 > As for the first issue - the presence of the xmlns attribute in the
 > serialised document - I'd be interested to hear whether it is
 > considered acceptable to parse the serialised document and to find that
 > no non-null namespaceURI is set on the href element, given that such a
 > namespaceURI was set when the document was created.

The key issue: should the serialised-then-reparsed document have the 
same DOM "content" (XML InfoSet) if the user did not explicitly create 
the requisite namespace declaration attributes?

My answer: No, it should not be the same.
My reasoning: The user did not explicitly create the attributes
  => The DOM should not automagically create them (according to
 the L2 spec)
  => such attributes should not be serialised
   - The user didn't create them
   - The DOM implementation didn't create them
   - If the serialisation processor creates them, that gives the
 same end result as if the DOM impl had (wrongly) created them.
  => the serialisation is a faithful/naive representation of the
 (not-namespace-well-formed) DOM constructed by the user (who
 omitted required attributes).
  => The reloaded document is a different DOM to the original, i.e.
 it has a different infoset.

The xerces and jython snippet I posted the other day demonstrates this. 
If you look closely at that code, the actual DOM implementation and the 
serialisation processor used are from different libraries. The DOM is 
the inbuilt JAXP DOM implementation, Apache Crimson(the example only 
works on JDK 1.4). The serialisation processor is the Apache Xerces 
serialiser. The fact that the xmlns="DAV:" attribute didn't appear in 
the output document shows that BOTH the (Crimson) DOM implementation AND 
the (Xerces) serialiser chose NOT to automagically create the attribute.

If you run that snippet with other DOM implementations, by setting the 
"javax.xml.parsers.DocumentBuilderFactory" property, you'll find the 
same result.

Serialisation and namespace normalisation are both in the realm of DOM 
Level 3, whereas minidom is only L2 compliant. Automagically introducing 
L3 semantics into the L2 implementation is the wrong thing to do.

http://www.w3.org/TR/DOM-Level-3-LS/load-save.html
http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/namespaces-algorithms.html

[Paul Boddie]
 > In other words, ...
 >
 > What should the "Namespace is" message produce?

Namespace is None

If you want it to produce,

Namespace is 'DAV:'

and for your code to be portable to other DOM implementations besides 
libxml2dom, then your code should look like:-

 > document = libxml2dom.createDocument(None, "doc", None)
 > top = document.xpath("*")[0]
 > elem1 = document.createElementNS("DAV:", "href")

elem1.setAttributeNS(xml.dom.XMLNS_NAMESPACE, "xmlns", "DAV:")

 > document.replaceChild(elem1, top)
 > elem2 = document.createElementNS(None, "no_ns")

elem2.setAttributeNS(xml.dom.XMLNS_NAMESPACE, "xmlns", "")

 > document.xpath("*")[0].appendChild(elem2)
 > document.toFile(open("test_ns.xml", "wb"))

its-not-about-namespaces-its-about-automagic-ly'yrs,

-- 
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Re: Great books on Python?

[David Van Mosselbeen]

Tolga wrote:

> 
> I am not unfamiliar to programming but a newbie in Python. Could you
> recommend me (a) great book(s) to start with? Free online books or
> solid books are welcome.
> 
> Thanx in advance.

Some days ago there was an similar subject 'Learning Python', wish give you
some usefull informations a hope.

If you ask me for some greats book, a answer "O'Reilly !"

1) Learning Python
2) programming Python
3) Python Pocket Reference
4) Python in a Nuteshell
5) Python Essential Reference
6) Python Cookbook

Begin to buy the first ('1)') book, then second ('2)') ...

A fatal link that surelly help
http://nl.wikipedia.org/wiki/Python_%28programmeertaal%29 . I know it's in
Dutch language but there is a usefull link on the bottom of this page ... I
hope you think in same manner (some troubles to explain me), try and buy...
Buy if you like it and if you use it...I hope is clearly enought !!! 

N.B.: I have not make this page on Wikipedia.

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Re: ANN: pygene0.12 - Genetic Programming&Algorithms Library

[eXt]

aum wrote:
> Hi all,
> 
> This announcement supersedes an earlier announcement of pygene.
> 
> pygene 0.2 now supports genetic programming, in addition to the classical
> Mendelian genetic algorithms of the earlier version. I thank the
> respondents to the earlier announcement for inspiring me to implement GP
> functionality.
> 
> http://www.freenet.org.nz/python/pygene
Cool! I'll surly take a look at that :)

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Re: python and VisualFox dbf

[Thomas Ganss]

lux schrieb:
> Hi,
> I've a dfb written in VisualFox,
> I need to send a "pack" and "reindex"
> but in odbc is not supported...
> 
> Anybody know how to do this?
> 
> TIA,
> Luca
> 
I am quite sure this is supported in OLEDB.
I thought this was also supported via ODBC,
but you have to make sure that you are accessing
the dbf's exclusively.

HTH

thomas
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Re: information needed to make a connection between computers

[Tim Williams (gmail)]

On 11/12/05, John Walton <[EMAIL PROTECTED]> wrote:
 Hello again! I'm still working on that instant messenger
(for science fair), and I have been reading about networking in some
Java tutorials. In one part of it, it said to have a connection with
another computer, you need to know the IP name of the computer you want
to connect with. I don't know whether or not this is the same for
Python, but could someone please tell me what information of the
computer you want to connect with the you actually need for a
connection? In other words (or plain english), what information do I
need to get a connection with another computer (IP address, name, IP
name)? Also, could you tell me how to find it on a computer? Since I'll
be testing the instant messenger on the three computers in my house, I
just need to know how to find the information on the computer I'd
currently be on (not from any remote location). Thanks!  :)

You need to know the hostname of the remote machine you are trying to
make a connection to.    From this you can work out the
IP address of the machine and then connect to it
directly.    You could work with just the IP address,
but these are liable to change, where as the hostname in theory won't.

>>> import socket
>>> socket.gethostbyname('www.yahoo.com')
'216.109.117.207'
>>> 

If the machine is on your network,  using its name (eg 'JohnW'   )  should give the same result

HTH :)


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Re: Managing import statements

[Giovanni Bajo]

Shane Hathaway wrote:

> Here's the real problem: maintaining import statements when moving
> sizable blocks of code between modules is hairy and error prone.


You can also evaluate a solution like this:
http://codespeak.net/py/current/doc/misc.html#the-py-std-hook

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information needed to make a connection between computers

[John Walton]

 Hello again! I'm still working on that  instant messenger (for science fair), and I have been reading about  networking in some Java tutorials. In one part of it, it said to have a  connection with another computer, you need to know the IP name of the  computer you want to connect with. I don't know whether or not this is  the same for Python, but could someone please tell me what information  of the computer you want to connect with the you actually need for a  connection? In other words (or plain english), what information do I  need to get a connection with another computer (IP address, name, IP  name)? Also, could you tell me how to find it on a computer? Since I'll  be testing the instant messenger on the three computers in my house, I  just need to know how to find the information on the computer I'd  currently be on (not from any remote location). Thanks!  :)  -John  
	
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Re: binascii.crc32 results not matching

[Fredrik Lundh]

I wrote:

> this prints
>
> 0xF032519BL 0xF032519BL
> 0x90E3070AL 0x90E3070AL
>
> no time to sort out the int/long mess for binascii.crc32, but it pro-
> bably needs the same tweaking as PIL (which handles the CRC as
> two 16-bit numbers, to avoid sign hell).

I realized that I used 2.3 for testing.  under 2.4, the following snippet
runs without warnings:

import binascii

def test2(text, crc=0):
return hex((binascii.crc32(text, crc^-1)^-1)&0x)

in other words,

crc = binascii.crc32(text, crc^-1)^-1)&0x

should give the same result as larry's original CalculateCrc function.





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Re: Random Number Generation?

[[EMAIL PROTECTED]]

Dimos wrote:
> Hello All,
>
> I need some help with random number generation. What I
> need exactly is:
>
> To create a few thousand numbers, decimal and
> integers, between 5 and 90,
> and then to export them as a single column at a
> spreadsheet.
>
> I am newbie, I was not able to create decimals with
> the random modules of
> Python 2.3.

You use randint(a,b) to generate an integer between a and b.

For real numbers, the function is random(). But that result is
always between 0.0 and 1.0, so you have to make the range
adjustment yourself.

>>> import random
>>> for i in range(10):
print random.random()*85 + 5

20.2844473176
83.5690712033
77.3459998722
8.79906993754
53.3672450881
25.2609744882
19.8894951301
39.9794852838
43.4056977237
21.7770662903



>
> Thanks, Dimos
>
>
> __
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> Tired of spam?  Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com

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Re: Random Number Generation?

[Fredrik Lundh]

Dimos wrote:

> I need some help with random number generation. What I
> need exactly is:
>
> To create a few thousand numbers, decimal and
> integers, between 5 and 90,
> and then to export them as a single column at a
> spreadsheet.
>
> I am newbie, I was not able to create decimals with
> the random modules of
> Python 2.3.

the random function returns a random number between 0.0 and 1.0,
so

>>> import random
>>> help(random.random)
Help on built-in function random:

random(...)
random() -> x in the interval [0, 1).

so to get what you want, all you have to do is to multiply by (90-5)
and add 5 at the end:

v = random.random() * 85.0 + 5.0

the random.uniform() wrapper does exactly this:

v = random.uniform(5.0, 90.0)





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Re: OO in Python? ^^

[Paul Boddie]

Steven D'Aprano wrote:
> I look at it this way: as the class designer, I have ZERO idea what
> attributes and methods of my class will be just the perfect thing to solve
> somebody's problem, so it is rude of me to lock them up as private --
> especially since they *will* find a way to hack my class and access my
> private attributes and methods anyway.

Actually, one good aspect of "attribute privacy" is the lowered risk of
instance attribute name clashes. When subclassing from some partly
abstract class in order to specialise its behaviour
(sgmllib.SGMLParser, for example), one has to make sure that one
doesn't reuse some instance attribute name by accident - doing so could
potentially cause behavioural issues in the superclass's mechanisms.
That said, Python does at least let you look at which attributes are
already defined in an instance due to the lack of privacy, so it does
offer some kind of solution to that problem. Moreover, Python also lets
you define double-underscore attribute names which behave give instance
attributes privacy in all respects, being invisible to users of the
instances concerned, accessible only within methods belonging to the
defining class, and safely overloadable in subclasses without incurring
conflicts.

Generally, I'm in agreement with you, though: private, protected and
final are all things which provide easy distractions from more serious
and demanding issues. They can quite often prove to be "the bikeshed"
in system design and implementation.

Paul

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Re: OO in Python? ^^

[Xavier Morel]

Matthias Kaeppler wrote:
> Why would I want to use an attribute in Python, where I would use 
> getters and setters in Java? I know that encapsulation is actually just 
> a hack in Python (common, "hiding" an implementation detail by prefixing 
> it with the classname so you can't access it by its name anymore? Gimme 
> a break...), but is that a reason to only write white box classes? ^^
> 
> - Matthias
> 

If you've ever written non-trivial code in Java, you can't deny that 
most of the classes are littered by pages and pages of

--
protected FooClass foo;
FooClass getFoo() {
return foo;
}
void setFoo(FooClass foo) {
this.foo = foo;
}
--

This is more or less equivalent to a simple
--
public FooClass foo;
--
but allows you to change the implementation details of the class without 
having to bork your whole interface later.

Now, you have no reason to do this in python, you can use a regular 
"real" attribute, and if you need to change the  implementation details, 
you can remove the real attribute and replace it with a virtual 
attribute through properties without changing the class' interface.
--
 >>> class Bar(object):
def __init__(self):
self.foo = 5


 >>> bar = Bar()
 >>> bar.foo
5
 >>>
 >>> # now let's change the Bar class implementation and split foo in 
boo and far
 >>>
 >>> class Bar(object):
def __init__(self):
self.boo = 2
self.far = 3
def _getfoo(self):
return self.boo + self.far
foo = property(_getfoo)


 >>> bar = Bar()
 >>> bar.foo
5
--
And this is completely transparent for anyone using the Bar class, they 
don't give a damn about what happens inside.

You can also use it to add checks on the allowed values, for example
--
 >>> class Bar(object):
def __init__(self):
self._foo = 5
def _getfoo(self):
return self._foo
def _setfoo(self,foo):
if not 0 <= foo <= 10:
raise ValueError("foo's value must be between 0 and 10 
(included)")
self._foo = foo
foo = property(_getfoo, _setfoo)


 >>> bar = Bar()
 >>> bar.foo
5
 >>> bar.foo = 2
 >>> bar.foo
2
 >>> bar.foo = 20

Traceback (most recent call last):
   File "", line 1, in -toplevel-
 bar.foo = 20
   File "", line 8, in _setfoo
 raise ValueError("foo's value must be between 0 and 10 (included)")
ValueError: foo's value must be between 0 and 10 (included)
 >>>
--
or everything else you'd use Java's getters/setters for, but without 
having to over-engineer your classes and wrap everything just because 
the language doesn't allow you to change your mind if you ever realize 
you made mistakes in the previous implementations.
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Re: Random Number Generation?

[Alex Martelli]

Dimos <[EMAIL PROTECTED]> wrote:

> Hello All,
> 
> I need some help with random number generation. What I
> need exactly is:
> 
> To create a few thousand numbers, decimal and
> integers, between 5 and 90, 
> and then to export them as a single column at a
> spreadsheet.
> 
> I am newbie, I was not able to create decimals with
> the random modules of Python 2.3.

The random module lets you create floats and integers, not instances of
class decimal.Decimal (surely not in 2.3, which didn't even HAVE a
module decimal in its standard library!).  If you want floats, the way
to create a float with uniform distribution between 5 and 90 is to call
random.uniform(5, 90).  If you want 3000:

r3k = [random.uniform(5, 90) for i in xrange(3000)]

None of the 3k numbers will be integers, and it's unlikely that any of
the 3k floats happens to have a fractional part that's exactly 0.  If
you do want integers as well as floats, you'll have to decide with what
probability an integer must appear instead of a float, and do a second
pass on r3k to enforce this.


Alex
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Re: OO in Python? ^^

[Alex Martelli]

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

> On Sun, 11 Dec 2005 17:05:16 +0100, Matthias Kaeppler wrote:
> 
> > Why would I want to use an attribute in Python, where I would use 
> > getters and setters in Java? 
> 
> Oh boy! I've just come out of a rather long thread about that very issue.
> If you care enough to read a bunch of people arguing past each other,
> check the thread "Another newbie question", especially the posts about the
> so-called Law of Demeter.
> 
> But for the short summary: suppose I write a class:
> 
> class Parrot:
> def __init__(self, x):
> self._x = x
> print "please use instance.getx() and setx()"
> def getx(self):
> return self._x
> def setx(self, x):
> self._x = x
> 
> What if I want to export methods of x other than get and set? Perhaps x
> is a numeric value: now I have to export a whole series of arithmetic
> operators: addx, subx, mulx, etc. And there may be other attributes I need
> to export as well...

Sorry, I don't see this.

a.setx( b.getx() + c.getx() - d.getx() )

will work just as well as (in a better-designed class) would

a.x = b.x + c.x - d.x

It's just that the former style you force syntactic cruft and overhead
which you may save in the latter.  "Exporting a series of operators",
which was an issue in the LOD thread, is not one here: once you have
setter and getter, by whatever syntax, it's not germane.


Alex
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Re: binascii.crc32 results not matching

[Larry Bates]

Thanks so much for the offer, I had a friend do this for
me and it works great.

Regards,
Larry Bates

Heiko Wundram wrote:
> Larry Bates wrote:
> 
>>
>>
>>The algorithm looks very much like the source code for
>>binascii.crc32 (but I'm not a C programmer).
> 
> 
> Well... As you have access to the code, you might actually just create a
> thin Python-Wrapper around this so that you can get comparable results. In
> case you're unable to do so, send me the C-file (I'm not so keen on
> copy-pasting code which was reformatted due to mail), and I'll send you an
> extension module back.
> 
> --- Heiko.
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Re: OO in Python? ^^

[Alex Martelli]

Matthias Kaeppler <[EMAIL PROTECTED]> wrote:
   ...
> I'm so used to statically typed languages that the shift is very 
> confusing. Looks as if it isn't as easy to learn Python afterall, for
> the mere reason of unlearning rules which don't apply in the world of
> Python anymore (which seem to be quite a lot!).

If you start your reasoning with Java for most aspects (and with C++ for
just a few) it may get easier -- for example, garbage collection, the
semantics of assignment and argument passing, as well as the
immutability of strings, are very close in Java and Python, and they're
such a "bedrock layer" parts of the languages that IMHO they're more
likely to be stumbling blocks if you "think C++" rather than "think
Java", while learning Python.

If you can imagine a Java program where everything is declared to be
Object, all method calls have an implicit cast to "some interface
supplying this method", and all class declarations implicitly add many
"implements IMethodFoo" for automatic interfaces supplying each of their
methods foo, you're a good part of the way;-).  Such a style would be
unusual (to say the least) in Java, but it's clearly POSSIBLE... it's
actually not that unusual in Objective C (except you say ID rather than
Object, and implicit interfaces ARE sort of there) -- the main caveat i
would give to an Objective C person learning Python (besides the garbage
collection issues) is "in Python, you cannot call arbitrary methods on
None and expect them to be innocuous noops" (like in Java or C++,
calling anything on None, aka a NULL pointer aka null, is a runtime
error in Python too; the language without this restriction in this case
is Objective C!-).


Alex
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Re: OO in Python? ^^

[Alex Martelli]

Matthias Kaeppler <[EMAIL PROTECTED]> wrote:
   ...
> I stumbled over this paragraph in "Python is not Java", can anyone 
> elaborate on it:
> 
> "In Java, you have to use getters and setters because using public 
> fields gives you no opportunity to go back and change your mind later to
> using getters and setters. So in Java, you might as well get the chore
> out of the way up front. In Python, this is silly, because you can start
> with a normal attribute and change your mind at any time, without 
> affecting any clients of the class. So, don't write getters and setters."
> 
> Why would I want to use an attribute in Python, where I would use 
> getters and setters in Java? I know that encapsulation is actually just
> a hack in Python (common, "hiding" an implementation detail by prefixing
> it with the classname so you can't access it by its name anymore? Gimme
> a break...), but is that a reason to only write white box classes? ^^

Consider the difference between asking a user of your class to write:

a.setFoo(1 + a.getFoo())

versus allowing him or her to write:

a.foo += 1

I hope you can see how much more elegant and handy the second
alternative is.  The point is, in Python, you can easily make the
semantics of the second alternative identical to those of the first: all
you need to do is, within a's class, add ONE line:
foo = property(getFoo, setFoo)
and every access of a.foo will become a call to a.getFoo(), every
setting of a.foo=bar a call to a.setFoo(bar).

The ability to do this IF AND WHEN getFoo and setFoo really need to
exist (i.e., they do something meaningful, not just boilerplate setting
and getting of plain attributes) empowers you to always allow the users
of your class to access attributes -- you will change an attribute to a
property only in future versions of your class that do need some
meaningful action upon the getting or setting or that attribute.


Alex
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Random Number Generation?

[Dimos]

Hello All,

I need some help with random number generation. What I
need exactly is:

To create a few thousand numbers, decimal and
integers, between 5 and 90, 
and then to export them as a single column at a
spreadsheet.

I am newbie, I was not able to create decimals with
the random modules of 
Python 2.3.

Thanks, Dimos 


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Re: OO in Python? ^^

[Aahz]

In article <[EMAIL PROTECTED]>,
Matthias Kaeppler  <"matthias at finitestate dot org"> wrote:
>
>Why would I want to use an attribute in Python, where I would use 
>getters and setters in Java? I know that encapsulation is actually just 
>a hack in Python (common, "hiding" an implementation detail by prefixing 
>it with the classname so you can't access it by its name anymore? Gimme 
>a break...), but is that a reason to only write white box classes? ^^

"Simple is better than complex."
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Re: OO in Python? ^^

[Steven D'Aprano]

On Sun, 11 Dec 2005 17:05:16 +0100, Matthias Kaeppler wrote:

> Why would I want to use an attribute in Python, where I would use 
> getters and setters in Java? 

Oh boy! I've just come out of a rather long thread about that very issue.
If you care enough to read a bunch of people arguing past each other,
check the thread "Another newbie question", especially the posts about the
so-called Law of Demeter.

But for the short summary: suppose I write a class:

class Parrot:
def __init__(self, x):
self._x = x
print "please use instance.getx() and setx()"
def getx(self):
return self._x
def setx(self, x):
self._x = x


What if I want to export methods of x other than get and set? Perhaps x
is a numeric value: now I have to export a whole series of arithmetic
operators: addx, subx, mulx, etc. And there may be other attributes I need
to export as well...

When I write getter and setter methods for an attribute, I make myself
responsible for maintaining everything about that attribute. I create
extra functions that need to be tested, debugged and documented. I expect
my class users to learn how to use my class, and every method is another
method they have to learn about. Writing getter and setter methods have
costs -- I pay some of those costs, and my users pay some of those costs.

Then I get fed up and write this class instead:

class NorwegianBlue:
def __init__(self, x):
self.x = x
print "please use public attribute instance.x"

NorwegianBlue is a much smaller class. That means less development time
for me, less test code to write, less documentation, less time needed for
my users to learn the API -- they already know what the API for x is,
because they supplied it. The costs to both class designer and class
user are much less, the flexibility is greater, and I finish writing the
class quicker.

Is there a disadvantage to NorwegianBlue? Yes -- I am now locked in to one
particular interface. I can't even do something as trivial as change the
name of attribute x. But then I can't change the name of Parrot.getx
either, not without breaking people's code.

But still, sometimes I know that I will want to -- or even that I might
want to -- radically change the implementation of my class. Perhaps x is a
list now, but later it will be a binary tree. How much effort will be
needed to fix the breakage if NorwegianBlue changes? If the expected
effort to fix the breakage is more than the effort to write setters and
getters, then it makes sense to write setters and getters. 

But contrariwise, if it will take less effort to fix the problem than to
defend against it, then why defend against it? Getters and setters are
insurance against you changing the private interface. You wouldn't pay
$20,000 a year to protect a $20,000 car. You might not even pay $1,000 a
year. 

So do your trade-offs, and make your decision.



> I know that encapsulation is actually just 
> a hack in Python (common, "hiding" an implementation detail by prefixing 
> it with the classname so you can't access it by its name anymore? Gimme 
> a break...), 

I'll give you a much better break in C++:

#def private = public

So much for encapsulation, hey?

Java makes it harder, but not that much more:

http://www.informit.com/articles/article.asp?p=174217&seqNum=3&rl=1



> but is that a reason to only write white box classes? ^^

Why would you want to write black box classes?

Python is not a bondage and discipline language. Remember kids, whips and
chains are only for mummies and daddies who love each other very much.

Python includes tools for preventing people *accidentally* shooting
themselves in the foot. But the philosophy of the language is to allow,
even to encourage, people to tinker under the hood. If that means that
somebody *deliberately* shoots themselves in the foot, well, that's the
price you pay for freedom: some people make stupid decisions.

I look at it this way: as the class designer, I have ZERO idea what
attributes and methods of my class will be just the perfect thing to solve
somebody's problem, so it is rude of me to lock them up as private --
especially since they *will* find a way to hack my class and access my
private attributes and methods anyway. All I'm doing is making their life
miserable by making them go out and buy books like "Hacking Private
Attributes In Java" or something.

But as a class designer, I know perfectly well which of my class members
are free for anyone to touch (instance.public), which should be considered
private by convention (instance._private) and which need a great big sign
on the door saying "Enter At Own Risk! Don't touch this unless you know
what you are doing!!!" (instance.__mangled).



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Re: XML and namespaces

[Paul Boddie]

Alan Kennedy wrote:

[Discussing the appearance of xmlns="DAV:"]

> But that's incorrect. You have now defaulted the namespace to "DAV:" for
> every unprefixed element that is a descendant of the href element.

[Code creating the no_ns element with namespaceURI set to None]

> 
> 

I must admit that I was focusing on the first issue rather than this
one, even though it is related, when I responded before. Moreover,
libxml2dom really should respect the lack of a namespace on the no_ns
element, which the current version unfortunately doesn't do. However,
wouldn't the correct serialisation of the document be as follows?




As for the first issue - the presence of the xmlns attribute in the
serialised document - I'd be interested to hear whether it is
considered acceptable to parse the serialised document and to find that
no non-null namespaceURI is set on the href element, given that such a
namespaceURI was set when the document was created. In other words, ...

document = libxml2dom.createDocument(None, "doc", None)
top = document.xpath("*")[0]
elem1 = document.createElementNS("DAV:", "href")
document.replaceChild(elem1, top)
elem2 = document.createElementNS(None, "no_ns")
document.xpath("*")[0].appendChild(elem2)
document.toFile(open("test_ns.xml", "wb"))

...as before, followed by this test:

document = libxml2dom.parse("test_ns.xml")
print "Namespace is", repr(document.xpath("*")[0].namespaceURI)

What should the "Namespace is" message produce?

Paul

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Re: instance + classmethod question

[Laszlo Zsolt Nagy]

  Hello Steven,

I already implemented this using the form

@classmethod
def  methodname(cls,other_params,self=None)

but your example code looks so neat! This is exactly what I needed. :-)
In my methods, most code is about string manipulation and calling other 
classmethods.
There are only a few places where I can use an instance, but it is not 
required.
I would like to reuse as most code as possible, so I do not want to 
create two different
methods. That would result in duplicating code. Now the only problem is 
how I name this.
It is not a classmethod, but it is also not a normal method. All right, 
it is a
"ClassOrInstanceMethod". Amazing! Probably Python is the only language 
that is
flexible enough to do this. :-)

Thanks again!

   Laszlo


Steven Bethard wrote:

>Laszlo Zsolt Nagy wrote:
>  
>
>> Hello,
>>
>>Is it possible to tell, which instance was used to call the classmethod 
>>that is currently running?
>>
>>
> >>> class ClassOrInstanceMethod(object):
>... def __init__(self, func):
>... self.func = func
>... self.classfunc = classmethod(func)
>... def __get__(self, obj, objtype=None):
>... func = obj is None and self.classfunc or self.func
>... return func.__get__(obj, objtype)
>...
>  
>

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Re: Great books on Python?

[Nate Bargmann]

On Sun, 11 Dec 2005 06:15:17 -0800, Tolga wrote:

> 
> I am not unfamiliar to programming but a newbie in Python. Could you
> recommend me (a) great book(s) to start with? Free online books or
> solid books are welcome.
> 
> Thanx in advance.

O'Reilly's Learning Python Second Edition covers up to version 2.3 and
presumes a bit of knowledge with C.  I've found it well written with a
rather low count of errors.

- Nate >>

-- 

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

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Re: OO in Python? ^^

[Paul Boddie]

Steven D'Aprano wrote:
> Weakly typed languages do not prevent you performing operations on
> mismatched types, e.g. something like 1 + "1" is allowed in languages like
> Basic and Perl.

Actually, Perl and at least the version of BASIC that I previously used
are not weakly-typed languages either. The addition operator in Perl
uses coercion much as Python does/supports for some operand types, and
classic microcomputer BASICs generally suffixed variable names in order
to impose some kind of static typing. One classic example of a
weakly-typed language is BCPL, apparently, but hardly anyone has any
familiarity with it any more.

Paul

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Re: OO in Python? ^^

[bonono]

Steven D'Aprano wrote:
> On Sun, 11 Dec 2005 07:10:27 -0800, bonono wrote:
>
> >
> > Steven D'Aprano wrote:
> >> > And I don't think Haskell make the programmer do a lot of work(just
> >> > because of its static type checking at compile time).
> >>
> >> I could be wrong, but I think Haskell is *strongly* typed (just like
> >> Python), not *statically* typed. At least the "What Is Haskell?" page at
> >> haskell.org describes the language as strongly typed, non-strict, and
> >> allowing polymorphic typing.
> >>
> > What is your definition of statically typed ? The non-strict as far as
> > I know is not referring to type checking. It does check type at compile
> > time though it is quite different from language like C, Java, the
> > polymorphic typing.
>
> Strongly typed means that objects have a type. All objects in Python have
> a type.
>
> Strongly typed languages like Python forbid you from performing operations
> on mismatched types, e.g. 1 + "1" does not work. In order to perform
> operations on mismatched types, you must explicitly perform a conversion,
> e.g. 1 + int("1").
>
> Weakly typed languages do not prevent you performing operations on
> mismatched types, e.g. something like 1 + "1" is allowed in languages like
> Basic and Perl.

This much I know but it was not what we are talking about.

>
> Untyped languages do not have any type information at all -- everything
> is just bytes. The most obvious example is assembly language.
>
> It should be noted that strong and weak typing is a matter of degree:
> despite being mostly strongly typed, Python does do automatic coercion of
> ints and floats, and although it is (arguably) weakly typed, Perl won't
> allow you to treat scalars as arrays or vice versa.
>
> Dynamic typing means that variables can be dynamically set to objects of
> wildly different types. For Python, we would say that any name can be
> bound to any object of any type.
>
> Static typing is the opposite of dynamic typing. Once a variable or
> name is defined as a certain type (either by a declaration, or
> implicitly the first time it is used), it can only be assigned to values
> of that same type.
>
> These two articles may be helpful:
>
> http://www.voidspace.org.uk/python/articles/duck_typing.shtml
> http://www.artima.com/forums/flat.jsp?forum=106&thread=7590
>
>
> A thoughtful defence of static typing is here:
>
> http://www.xoltar.org/misc/static_typing_eckel.html
>
> The fact that it is sub-titled "How Java/C++/C# Ruin Static Typing for the
> Rest of Us" should give some idea what it is about.
>
And you would see in the xoltar link that Haskell is a language of
static typing. You cannot call a function with parameters with
incompatible types which would be checked at compile time.

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Re: OO in Python? ^^

[Paul Boddie]

Heiko Wundram wrote:
> Maybe I'm assuming things by thinking that others also follow my line of
> thought, but I've actually had very positive responses so far when telling
> people that a certain feature is a certain way and then pointing them to
> the ZoP, they all pretty much told me after a certain time of thought that
> "the decision made sense now."

Sorry to come across all harsh on the subject. Perhaps you know people
who are more meditative than I do, but I notice that you served up some
concrete advice elsewhere in the thread, so if the ZoP doesn't provide
any guidance to the questioner as it is, at least there's something
else to read through and grasp.

Paul

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Re: ANN: Dao Language v.0.9.6-beta is release!

[Fredrik Lundh]

Zeljko Vrba wrote:

> Nobody bothers to figure out something better? Now you will argue that then
> the indendation is good enough.. and I can just reply that then it's an
> open research question..

huh?  people mention existing research (including formal usability studies),
and your response is that since you don't agree with the results, more re-
search is needed.

do you think this is a kansas school board meeting?

> I have no problem accepting that I'm in a minority. I have a problem with
> offensive people using my example arguments to riducule me..

http://www.google.com/search?q=duck+typing





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Re: OO in Python? ^^

[Steven D'Aprano]

On Sun, 11 Dec 2005 07:10:27 -0800, bonono wrote:

> 
> Steven D'Aprano wrote:
>> > And I don't think Haskell make the programmer do a lot of work(just
>> > because of its static type checking at compile time).
>>
>> I could be wrong, but I think Haskell is *strongly* typed (just like
>> Python), not *statically* typed. At least the "What Is Haskell?" page at
>> haskell.org describes the language as strongly typed, non-strict, and
>> allowing polymorphic typing.
>>
> What is your definition of statically typed ? The non-strict as far as
> I know is not referring to type checking. It does check type at compile
> time though it is quite different from language like C, Java, the
> polymorphic typing.

Strongly typed means that objects have a type. All objects in Python have
a type.

Strongly typed languages like Python forbid you from performing operations
on mismatched types, e.g. 1 + "1" does not work. In order to perform
operations on mismatched types, you must explicitly perform a conversion,
e.g. 1 + int("1").

Weakly typed languages do not prevent you performing operations on
mismatched types, e.g. something like 1 + "1" is allowed in languages like
Basic and Perl.

Untyped languages do not have any type information at all -- everything
is just bytes. The most obvious example is assembly language.

It should be noted that strong and weak typing is a matter of degree:
despite being mostly strongly typed, Python does do automatic coercion of
ints and floats, and although it is (arguably) weakly typed, Perl won't
allow you to treat scalars as arrays or vice versa.

Dynamic typing means that variables can be dynamically set to objects of
wildly different types. For Python, we would say that any name can be
bound to any object of any type.

Static typing is the opposite of dynamic typing. Once a variable or
name is defined as a certain type (either by a declaration, or
implicitly the first time it is used), it can only be assigned to values
of that same type.

These two articles may be helpful:

http://www.voidspace.org.uk/python/articles/duck_typing.shtml
http://www.artima.com/forums/flat.jsp?forum=106&thread=7590


A thoughtful defence of static typing is here:

http://www.xoltar.org/misc/static_typing_eckel.html

The fact that it is sub-titled "How Java/C++/C# Ruin Static Typing for the
Rest of Us" should give some idea what it is about.


-- 
Steven.

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Re: Another newbie question

[Alex Martelli]

Mike Meyer <[EMAIL PROTECTED]> wrote:
   ...
> > Claim: doing X makes Y hard.
> 
> Harder, not hard.

The specific wording you used was "MORE DIFFICULT".


> > Here is an example of doing X where Y is easy
> 
> Y is very easy in any case. Making it incrementally harder doesn't
> make it hard - it's still very easy.

If it's very easy, then going out of your way, as you did, to claim it's
"MORE DIFFICULT" (you did not use the words "incrementally harder") is
rather weird.  There's no DIFFICULTY -- sure, if you have ANY one extra
trivial method there IS ``extra work'' (a few seconds to write the
trivial method and its unittest), but no extra DIFFICULTY.

Moreover, I believe I vindicated attribute-setters (and their lack of
difficulty) specifically by pointing out the trivial pattern which lets
you make them easily, by using a constructor that you must have anyway
in a good design (if attributes are worth setting on the fly, they're
worth setting at the birth of an instance) and a state-copying method
(which is always a good idea for mutable-instance classes).  Assuming
this wasn't obvious at the start to all readers, I may thus have hoped
to have taught something to some reader -- an unusual and pleasant
fallout from a mostly "polemical" thread, since often such threads are
not very instructive.

On that vein, I'll continue by pointing out that there may well be
opportunities for optimization -- constructing a new instance is easy,
but in some cases, depending on the implementation details, there may be
faster approaches.  That's most of the case for our using languages with
modifiable data rather than pure functional ones, after all...: that
changing data (rather than always having to make new objects) sometimes
affords better performance.  Still, let's not optimize *prematurely*!-)


Alex
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Re: instance + classmethod question

[Steven Bethard]

Laszlo Zsolt Nagy wrote:
> 
>  Hello,
> 
> Is it possible to tell, which instance was used to call the classmethod 
> that is currently running?
> 
[snip]
> 
> processor = SQLProcessors.StdOutProcessor() # Print to stdout
> PostgreSQLConnection.process_create_tables(processor,dbdef)  # This 
> sould create all tables, using the processor
> 
> processor = SQLProcessors.DirectProcessor(conn) # Execute directly
> conn.process_create_tables(processor,dbdef) # This should create 
> non-existing tables only, using the processor
> 
> Is this possible?

It looks like you want a method that accepts either a class or an 
instance.  I would typically create two methods, one for the class-based 
table-creating, and one for the instance-based table-creating.  However, 
if you *have* to do it this way, you can introduce your own descriptor 
which should give you the behavior you want::

 >>> class ClassOrInstanceMethod(object):
... def __init__(self, func):
... self.func = func
... self.classfunc = classmethod(func)
... def __get__(self, obj, objtype=None):
... func = obj is None and self.classfunc or self.func
... return func.__get__(obj, objtype)
...
 >>> class C(object):
... @ClassOrInstanceMethod
... def f(*args):
... print args
...
 >>> C.f()
(,)
 >>> C().f()
(<__main__.C object at 0x00E73D90>,)

Basically, if the descriptor is called from a type (and thus ``obj is 
None``), we return a bound classmethod, and if the descriptor is called 
from an instance, we return a bound instance method.  Of course this now 
means you should write your code something like::

@ClassOrInstanceMethod
def process_create_tables(cls_or_self, processor, dbdef):
 ...

whose "cls_or_self" parameter gives me a bad code smell.  YMMV.

STeVe
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Re: OO in Python? ^^

[Matthias Kaeppler]

[EMAIL PROTECTED] wrote:
> Just follow the links.

I'll try ;-)

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Re: OO in Python? ^^

[Matthias Kaeppler]

gene tani wrote:
> http://naeblis.cx/rtomayko/2004/12/15/the-static-method-thing
> http://dirtsimple.org/2004/12/java-is-not-python-either.html
> http://dirtsimple.org/2004/12/python-is-not-java.html
> 
> http://idevnews.com/PrintVersion_CaseStudies.asp?Search3=web+services&Go2=Go&ID=118
> http://www.idevnews.com/PrintVersion_TipsTricks.asp?ID=107
> http://www.eros-os.org/pipermail/e-lang/2001-June/005337.html

First of all, thanks everybody for posting all these answers and links, 
much appreciated. What Bruce Eckel wrote about dynamic typing was quite 
convincing and reasonable.

I stumbled over this paragraph in "Python is not Java", can anyone 
elaborate on it:

"In Java, you have to use getters and setters because using public 
fields gives you no opportunity to go back and change your mind later to 
using getters and setters. So in Java, you might as well get the chore 
out of the way up front. In Python, this is silly, because you can start 
with a normal attribute and change your mind at any time, without 
affecting any clients of the class. So, don't write getters and setters."

Why would I want to use an attribute in Python, where I would use 
getters and setters in Java? I know that encapsulation is actually just 
a hack in Python (common, "hiding" an implementation detail by prefixing 
it with the classname so you can't access it by its name anymore? Gimme 
a break...), but is that a reason to only write white box classes? ^^

- Matthias


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Re: Another newbie question

[Alex Martelli]

Mike Meyer <[EMAIL PROTECTED]> wrote:
   ...
> It's not my cherished example - it actually came from someone

You picked it to (try and fail to) show that there is DIFFICULTY, which
I showed there isn't.

> else. That you can change the requirements so that there is no extra
> work is immaterial - all you've done is shown that there are examples
> where that don't require extra work. I never said that such examples
> didn't exist. All you've shown - in both the single concrete example
> and in a generalized case - is that any requirement can be changed so
> that it doesn't require any extra work. This doesn't change the fact
> that such cases exist, which is all that I claimed was the case.

Untrue: you claimed that the specific API (allowing attribute-setting)
"makes changing the object more difficult", not the obvious fact that
"there exist APIs so badly designed that they make changing more
difficult".  And I showed that, in the GENERAL case, since attributes
worth being made assignable are obviously also worth being made settable
in a constructor of some kind, having settable attributes doesn't and
cannot introduce any DIFFICULTY -- the API with settable attributes only
requires trivial methods, ones presenting no difficulty whatsoever,
which delegate all the real work to methods you'd need anyway
(particularly the obviously-needed constructor or factory).

So, I claim I have totally disproven your claims about difficulty
("extra work", as you're trying to weaselword your way out, might be
writing one or two trivial lines of code, but that's not DIFFICULT, and
the claim you originally made was about DIFFICULTY, not tiny amounts of
trivially easy "extra work" -- as I already mentioned, obviously ANY
method you add is "extra work" for you compared to not adding it, but
the interesting question is whether that entails any DIFFICULTY).

My claim hinges on the fact that constructors are important -- more so,
of course, for immutable instances, but even in the mutable case it's
pretty bad design if the ONLY way to have an instance in the state you
know is right is to make it in a state that's wrong and then call
mutators on it until its state is finally right... obviously it's
important to avoid imposing this busywork on all users of the class.  If
you further weaken your claim to "it's possible to design so badly that
everybody involved faces more work and difficulties", I'll most
obviously agree -- but such bad designs need not involve any
attribute-setters, nor does including attribute-setters imply or even
suggest that a design is bad in this way!


Alex
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Re: OO in Python? ^^

[bonono]

Ernst Noch wrote:
> Matthias Kaeppler wrote:
> > Brian Beck wrote:
> >
> >> def foo(self):
> >> raise NotImplementedError("Subclasses must implement foo")
> >
> >
> > That's actually a good idea, though not as nice as a check at
> > "compile-time" (jesus, I'm probably talking in C++ speech again, is
> > there such a thing as compile-time in Python at all?!)
> >
> > Another thing which is really bugging me about this whole dynamically
> > typing thing is that it seems very error prone to me:
> >
> > foo = "some string!"
> >
> > # ...
> >
> > if (something_fubar):
> >fo = "another string"
> >
> > Oops, the last 'o' slipped, now we have a different object and the
> > interpreter will happily continue executing the flawed program.
> >
> > I really see issues with this, can anyone comment on this who has been
> > working with Python more than just a day (like me)?
> >
> > Regards,
> > Matthias
>
> Matthias,
>
> maybe this article is of interest for you:
> http://www.mindview.net/WebLog/log-0025
>
And two related ones.

http://www.algorithm.com.au/mt/programming_languages/me_on_xoltar_on_static_typing.html

Just follow the links.

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Re: Great books on Python?

[D H]

Tolga wrote:
> I am not unfamiliar to programming but a newbie in Python. Could you
> recommend me (a) great book(s) to start with? Free online books or
> solid books are welcome.
> 


http://www.ibiblio.org/g2swap/byteofpython/read/index.html
http://www.ibiblio.org/obp/thinkCSpy/
http://www.freenetpages.co.uk/hp/alan.gauld/
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Re: Great books on Python?

[BartlebyScrivener]

http://www.awaretek.com/tutorials.html#regular

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Re: Great books on Python?

[Xavier Morel]

Tolga wrote:
> I am not unfamiliar to programming but a newbie in Python. Could you
> recommend me (a) great book(s) to start with? Free online books or
> solid books are welcome.
> 
> Thanx in advance.
> 
I'd call Dive Into Python a reference, it's an extremely clear yet 
pythonic book, and it's available online for free.

And I guess that you already checked it, but the Python's Tutorial is -- 
of course -- imperative.
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Re: OO in Python? ^^

[D H]

Fredrik Lundh wrote:
> Write code, not usenet posts.

QOTW!
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Re: subset permutations

[Anton Vredegoor]

Steven D'Aprano wrote:

> On Fri, 09 Dec 2005 16:03:46 +1100, Ben Finney wrote:
>
> >> Do you want the result to be:
> >> AB, AC, AD, BC, BD, CD
> >
> > That is the complete set of combinations of the letters.
> >
> >> Or, do you want AB,BA,AC,CA,AD,DA,BC,CB,BD,DB,CD,DB ?
> >
> > That is the complete set of permutations of the letters.
>
>
> Only if you are choosing without replacement. If you are choosing with
> replacement, you also get AA, BB, CC, DD.
>
> Unfortunately, there is ambiguity in the terms "permutation" and
> "combination", not just between English common usage and mathematics, but
> even in mathematics.

Why is that unfortunate? Now we can all post our favorite scripts and
let them be severely criticized :-)

Anton

def ncycle(seq,n):
while True:
for x in seq:
i = 0
while i < n:
yield x
i += 1

def cross(*args):
p = 1
R = []
for seq in reversed(args):
R.append(ncycle(seq,p))
p *= len(seq)
R.reverse()
i = 0
while i < p:
yield tuple(x.next() for x in R)
i += 1

def test():
s1='a1','a2','a3','a4'
s2='b1','b2'
s3='c1','c2','c3'
for x in cross(s1,s2,s3):
print x

if __name__=='__main__':
test()

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instance + classmethod question

[Laszlo Zsolt Nagy]

  Hello,

Is it possible to tell, which instance was used to call the classmethod 
that is currently running?

Background: I have a class called DatabaseConnection and it has a 
classmethod called process_create_tables. This method should create some 
database tables defined by a database definition object. The 
DatabaseConnection has many descendants, for example 
PostgreSQLConnection. Descendants know how to create tables in a given 
RDBMS type. I also use subclasses of the 'SQLProcessor' class, that 
processes SQL commands in different ways (print to stdout, write to 
file, execute directly in the database etc.) I would like to use the 
process_create_tables classmethod as is, because sometimes I only need 
to save a SQL script. However, I also want to use the same classmethod 
to create tables directly into an existing database. That database is 
presented as a DatabaseConnection instance. In that case, I only want to 
create the tables that do not exists yet.  Examples:

processor = SQLProcessors.StdOutProcessor() # Print to stdout
PostgreSQLConnection.process_create_tables(processor,dbdef)  # This 
sould create all tables, using the processor

processor = SQLProcessors.DirectProcessor(conn) # Execute directly
conn.process_create_tables(processor,dbdef) # This should create 
non-existing tables only, using the processor

Is this possible? Maybe there is a better way to achieve this, I'm not 
sure. I was thinking about this construct:

@classsmethod
def process_create_tables(cls,processor,dbdef,conn=None)

and then calling it as

conn.process_create_tables(processor,dbdef,conn)

but this looks very ugly to me. It would be much easier if I could tell 
which instance (if any) was used to call the classmethod.

Thanks,

   Les

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Re: OO in Python? ^^

[bonono]

Steven D'Aprano wrote:
> > And I don't think Haskell make the programmer do a lot of work(just
> > because of its static type checking at compile time).
>
> I could be wrong, but I think Haskell is *strongly* typed (just like
> Python), not *statically* typed. At least the "What Is Haskell?" page at
> haskell.org describes the language as strongly typed, non-strict, and
> allowing polymorphic typing.
>
What is your definition of statically typed ? The non-strict as far as
I know is not referring to type checking. It does check type at compile
time though it is quite different from language like C, Java, the
polymorphic typing.

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Re: OO in Python? ^^

[Steven D'Aprano]

On Sun, 11 Dec 2005 05:48:00 -0800, bonono wrote:

> 
> Steven D'Aprano wrote:
>> Python works well with test-driven development. Test-driven development
>> will pick up this sort of error, and many other errors too, with less
>> effort and more certainty than compile-time checking. The problem with
>> static typed languages is that they make the programmer do a lot of the
>> work, just so the compiler can pick up a few extra errors at compile time
>> rather than at run time.
>>
> Any language would be benefited from test-driven development, python
> needs it because of its dynamic nature.

We can use "need" in the strict sense, as in "the language won't work
without it". I think we can reject that as too strong, because clearly
Python can work without unittests or any other sort of testing.

In the looser sense of "this will benefit you", I think it is fair to say
that *all* languages need test-driven development. If you want your code
to do some non-trivial task X, and you don't actually test to see if
it really does do X, then all the compiler tests in the world won't tell
you that your code is doing X. 

Of course, the IT world is full of people writing code and not testing
it, or at least not testing it correctly. That's why there are frequent
updates or upgrades to software that break features that worked in the
older version. That would be impossible in a test-driven methodology, at
least impossible to do by accident.



> And I don't think Haskell make the programmer do a lot of work(just
> because of its static type checking at compile time).

I could be wrong, but I think Haskell is *strongly* typed (just like
Python), not *statically* typed. At least the "What Is Haskell?" page at
haskell.org describes the language as strongly typed, non-strict, and
allowing polymorphic typing.



-- 
Steven.

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Using XML w/ Python...

[Michael Williams]

If you just want to get into it and use it, I'd recommend the following:	http://uche.ogbuji.net/uche.ogbuji.net/tech/4suite/amara/It requires the installation of the 4Suite module as well, but it's well worth it.  I uses data binding techniques to convert your document into a large tree of named XML Object (for lack of a better explanation).  You will then be able to output any portion by calling something like the following:	document.body.text.xml()What you call, of course, will depend on the name of each of your nodes in the document.  Keep in mind as well that currently there are a couple of issues with "freezing" this into a binary as well.  If you just plan to let Python interpret it without "compiling" you will be fine.  Amara is an amazing product.  And being free doesn't hurt.  ;)Regards,MichaelOn Dec 11, 2005, at 1:15 AM, [EMAIL PROTECTED] wrote:OK, I have this  XML doc, i dont know much about XML, but what i want to do is take certain parts of the XML doc, such as  blah  and take just that and put onto a text doc. Then same thing doe the  part. Thats about it, i checked out some of the xml modules but dont understand how to use them. Dont get parsing, so if you could please explain working with XML and python to me. Email me at [EMAIL PROTECTED]  Aim- jayjay08balla MSN- [EMAIL PROTECTED] Yahoo- raeraefad72   Thx -- 
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Re: OO in Python? ^^

[Aahz]

In article <[EMAIL PROTECTED]>,
Matthias Kaeppler  <"matthias at finitestate dot org"> wrote:
>
>Another thing which is really bugging me about this whole dynamically 
>typing thing is that it seems very error prone to me:
>
>foo = "some string!"
>
># ...
>
>if (something_fubar):
>fo = "another string"
>
>Oops, the last 'o' slipped, now we have a different object and the 
>interpreter will happily continue executing the flawed program.

pychecker (or pylint, but I haven't tried that)
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