Re: Eval of expr with 'or' and 'and' within
On 15/6/2013 12:48 μμ, Lele Gaifax wrote: but those 2 gives the same results back "k" in (name+month+year) == "k" in (name and month and year) True so both seem to work as expected. That happens only by chance: it seems you now understand the evaluation of "boolean" expressions in Python, so the following should be clear to you: yes indeed! if we had questioned python for: "k" in (name and year and month) that would have returned the argument month back which is "efgh" and then the if would have evaled to false since 'k' isn't part of the latter. "k" in (name and month and year) != "k" in (name and year and month) As wee see, the order of the arguments in an expression matters. -- What is now proved was at first only imagined! -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 15/6/2013 12:48 μμ, Lele Gaifax wrote:
Nick the Gr33k writes:
but those 2 gives the same results back
"k" in (name+month+year) == "k" in (name and month and year)
True
so both seem to work as expected.
That happens only by chance: it seems you now understand the evaluation
of "boolean" expressions in Python, so the following should be clear to
you:
"k" in ("there" + "is" + "a" + "k" + "character" + "somewhere")
True
ALL strings in the parenthesis are being concatenated to a BIG string,
like ('k' in BIG_string) which returns the Boolean value of True since
'k' is contained within it.
"k" in ("there" and "is" and "a" and "k" and "character" and "somewhere")
False
first the expression of the parenthesis evaluation.
The argument being returned from ("there" and "is" and "a" and "k" and
"character" and "somewhere") expresssion, is the one that is responsible
for the determination of the expression's evaluation.
That would be the last argument, string "somewhere" because:
1. All previous strings before the last one were found truthies, so it
was turn of the last one to be evaled too.
2. Since "somewhere" is a truthy of course for being a non-empty string
then ALL arguments of the expression are found TRUE which results for
the expression to stand TRUE.
3. What determined the expression to stand TRUE was the evaluation of
the last argument "somewhere", so the latter was the key factor for that
to happen, hence its being returned back as a result(thats how Python works)
4. so it's like checking if ('k' in "somewhere") which return the
Boolean value of False since the aforementioned char isn't contained in
the aforementioned string.
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Re: Eval of expr with 'or' and 'and' within
Nick the Gr33k writes:
> On 15/6/2013 3:14 πμ, Cameron Simpson wrote:
>> But for what you are doing, "and" and "or" are not good operations.
>>
>> Something like:
>>
>>"k" in (name+month+year)
>>
>> or
>>
>>"k" in name or "k" in month or "k" in year
>
> Used to wrote it myself like the latter but needed a more compact way
> of writing it for clarity so i used the former.
>
> but those 2 gives the same results back
>
> "k" in (name+month+year) == "k" in (name and month and year)
> True
>
> so both seem to work as expected.
That happens only by chance: it seems you now understand the evaluation
of "boolean" expressions in Python, so the following should be clear to
you:
>>> "k" in ("there" + "is" + "a" + "k" + "character" + "somewhere")
True
>>> "k" in ("there" and "is" and "a" and "k" and "character" and "somewhere")
False
ciao, lele.
--
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
l...@metapensiero.it | -- Fortunato Depero, 1929.
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Re: Eval of expr with 'or' and 'and' within
On 15/6/2013 3:14 πμ, Cameron Simpson wrote:
On 14Jun2013 12:50, Nikos as SuperHost Support wrote:
| I started another thread because the last one was !@#$'ed up by
| irrelevant replies and was difficult to jeep track.
|
| >>> name="abcd"
| >>> month="efgh"
| >>> year="ijkl"
|
| >>> print(name or month or year)
| abcd
|
| Can understand that, it takes the first string out of the 3 strings
| that has a truthy value.
|
| >>> print("k" in (name and month and year))
| True
|
| No clue. since the expression in parenthesis returns 'abcd' how can
| 'k' contained within 'abcd' ?
Did you print the result of "name and month and year"? It is the
_last_ value (if true at all). You used "or" in the first example
and "and" in the second.
okey, lets see it again:
>>> print (name or month or year)
abcd
>>>
Yes, 'k' isn't contained in the result string 'abcd'
>>> print (name and month and year)
hijk
>>> print( "k" in (name and month and year) )
True
Yes they work as expected, i was mistaken, sorry.
| >>> print(name and month and year)
| ijkl
|
| Seems here is returning the last string out of 3 strings, but have
| no clue why Python doing this.
To evaluate an "and" it must test all of them to be true, and it
keeps the last value tested. (Or False, of course, if they are not
all true, in which case Python stops testing at the first False).
Yes, i know it behaves like that, the question is why:
As "Nobody" explained to me, the reason is that Python expressions
results back the argument that determined the evaluation of the Boolean
expression, which in turn can be a truthy or a falsey used in 'or' or
'and' respectively.
Returning a truthy value equals True
returning a falsey value equals False
so it all boils down to the Booleans type of values True or False.
But for what you are doing, "and" and "or" are not good operations.
Something like:
"k" in (name+month+year)
or
"k" in name or "k" in month or "k" in year
Used to wrote it myself like the latter but needed a more compact way of
writing it for clarity so i used the former.
but those 2 gives the same results back
"k" in (name+month+year) == "k" in (name and month and year)
True
so both seem to work as expected.
--
What is now proved was at first only imagined!
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Re: Eval of expr with 'or' and 'and' within
On 14/6/2013 7:42 μμ, Nobody wrote: Python implements these operators by returning the actual value which determined the result of the expression rather than simply True or False. which in turn the actual value being returned is a truthy or a falsey. That cleared the mystery in my head entirely. I wouldn't have asked so many follow-up questions in the thread if i received that kind of a response. Thank you very much for this response. If the result is known after evaluating the first argument, the first argument is returned. If it has to evaluate the second argument, the second argument is returned (by that point it has already forgotten the value of the first argument). So, the less it has to calculate to determine the correct result of an expression the better. Thanks again very much. -- What is now proved was at first only imagined! -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 15/6/2013 10:49 πμ, Denis McMahon wrote: On Sat, 15 Jun 2013 10:04:41 +0300, Nick the Gr33k wrote: I called my self 'Ferrous Cranus'(this is what a guy from a forum initially called me for being hard-headed :-) ) because i'm indeed hardheaded and if i believe that 1 thing should have worked in some way i cant change my mind easily that it works in another fashon(only if the counter-arguments are strong). Then you need to stop trying to write python code, because you refuse to accept how python works, and python is not going to change for you! Given the right counter-arguments one can't make a stand against something that is without no doubt True. -- What is now proved was at first only imagined! -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Sat, 15 Jun 2013 10:04:41 +0300, Nick the Gr33k wrote: > I called my self 'Ferrous Cranus'(this is what a guy from a forum > initially called me for being hard-headed :-) ) because i'm indeed > hardheaded and if i believe that 1 thing should have worked in some way > i cant change my mind easily that it works in another fashon(only if the > counter-arguments are strong). Then you need to stop trying to write python code, because you refuse to accept how python works, and python is not going to change for you! -- Denis McMahon, denismfmcma...@gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 15/6/2013 9:50 πμ, alex23 wrote: Please keep the snarky comments offlist. Tried that. He posts them back here. Alternatively, I'd ask that if you're so willing to deal with him, that the *two of you* take this show offlist instead? I'm genuinely curious as to whether he'd agree to this: given his propensity for changing his mail headers so regularly, the whole thing still screams performance troll. (I mean, seriously, how many days elapsed between his being burned by Chris Angelo and him offering to give you access to his server? Isn't refusing to budge from a position the defining characteristic of a ferrous cranus?) I called my self 'Ferrous Cranus'(this is what a guy from a forum initially called me for being hard-headed :-) ) because i'm indeed hardheaded and if i believe that 1 thing should have worked in some way i cant change my mind easily that it works in another fashon(only if the counter-arguments are strong). Being strong-headed != acting as a troll About the changing of NNTP hosts. (well Google Groups gave me and you issues with its constant adding of '\n' between lines. I was given many complains about that so as Chris suggested i ditched Google Groups and try to find out how i can use TB instead. As for the mail, i decided since my questions concern python and my website to change the gmail address to support@ which is more relevant. I have also correct each time my spelling before i post. Now as for you, you should be thankful for Steven and Cameron answers to this groups because answering my questions. 1. help me 2. help others, which afraid to ask, in the propensity of being laughed upon 3. help other groups members like Michale Torrie whose admitted that some of the explanation was useful to him and he didn't knew that Boolean expressions can return string values instead of True or False. 3. help you, because you are being educated yourself too, because i sincerely doubt if you knew anything i i have asked. So, stop complaining and try to be helpful here or just mute my thread as Chris suggested as a working solution opposed to kill filing. -- What is now proved was at first only imagined! -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Saturday, 15 June 2013 02:09:20 UTC+10, Steven D'Aprano wrote: > To everyone else... I know that Nikos' posts are draining. Sometimes he > brings me to the brink of despair too. But if you aren't part of the > solution, you are part of the problem: writing short-tempered, insulting > posts after short-tempered, insulting post doesn't teach him, it just > adds to EVERYBODY'S frustration with this never-ending serious of threads. What about our frustration that this thread has become so overwhelmingly his support group? It's not like there isn't a list where he could post absolute beginner questions: http://mail.python.org/mailman/listinfo/tutor Although I'm pretty sure their first response would be "read the docs / play with the interpreter", neither of which he's shown any desire to do. > Please keep the snarky comments offlist. Tried that. He posts them back here. Alternatively, I'd ask that if you're so willing to deal with him, that the *two of you* take this show offlist instead? I'm genuinely curious as to whether he'd agree to this: given his propensity for changing his mail headers so regularly, the whole thing still screams performance troll. (I mean, seriously, how many days elapsed between his being burned by Chris Angelo and him offering to give you access to his server? Isn't refusing to budge from a position the defining characteristic of a ferrous cranus?) -- http://mail.python.org/mailman/listinfo/python-list
NANs [was Re: Eval of expr with 'or' and 'and' within]
On Sat, 15 Jun 2013 12:03:08 +1000, Cameron Simpson wrote: > | ... even taking that into account! *wink* | > | Everyone is aware that there is more than one NAN, right? > > I was not. Interesting. > > | If my > | calculations are correct, there are 9007199254740992 distinct float > | NANs in Python (although there is no direct way of distinguishing > | them). > > Wouldn't id() do it? At least in terms of telling them apart? I gather > they're not inspectable in Python? I'm not talking about different *objects*. It would be terribly wasteful for Python to pre-allocate 9-gazillion NAN objects. I'm talking about distinct values, in the C-double sense. Python may or may not cache floats in general. In general, it doesn't: py> x = 2.5 py> y = 2.5 py> x is y False but in principle Python might choose to cache some, or all float objects, like it does with some ints and strings: # simulated, not real py> x = 0.0 py> y = 0.0 py> x is y True So you can always distinguish two floats, including NANs, *by value* with == and by *object identity* with id() and `is`. But that's not what I'm referring to. The layout of a C double (the numeric value of a float object) is 64 bits: |s|..e..|..f..| where s is a single sign bit, e is an 11-bit biased exponent, and f is a 52-bit binary fraction. If e is 2047 (0b111) then the double is a special value, either an INF or a NAN: e = 2047, f == 0: double is +INF or -INF, depending on s e = 2047, f != 0: double is a NAN With a 52-bit f field, there are 4503599627370496 distinct payloads with the sign bit set, and another 4503599627370496 with it cleared. Python gives you no direct way of testing or setting that payload, or for that matter the sign bit, on a NAN, although you can use the struct module to cast a float to a 64-bit int and then do bit twiddling. But in principle, with 51 (why not 52? see below) bits available, you can stuff quite a fair bit of diagnostic information in a NAN, if you need to. The high bit of f is reserved for a special purpose. If the high bit is set (i.e. f >> 51 == 1) the NAN is considered a signalling NAN, which (in implementations that comply with the IEEE 754 standard) are guaranteed to halt the calculation immediately. Those with it cleared are quiet NANs, and are intended to propagate through calculations[1], although that is configurable[2]. [1] It's not quite true that once a NAN has entered a calculation, it is guaranteed to be the final result. There are circumstances where NANs can legitimately disappear from a calculation, leaving an actual number. [2] Python doesn't allow you to configure float's behaviour but the Decimal module does. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Sat, Jun 15, 2013 at 12:03 PM, Cameron Simpson wrote:
> On 15Jun2013 01:34, Steven D'Aprano
> wrote:
> | Everyone is aware that there is more than one NAN, right?
>
> I was not. Interesting.
>
> | If my
> | calculations are correct, there are 9007199254740992 distinct float NANs
> | in Python (although there is no direct way of distinguishing them).
>
> Wouldn't id() do it? At least in terms of telling them apart?
> I gather they're not inspectable in Python?
You could recognize one float object as distinct from another, but
that's true of all floats:
>>> float("1.0") is float("1.0")
False
All NaNs are different in terms of the == operator, so conceptually
there are an infinite number of unique NaNs. The fact that they're
stored in memory using a certain number of bits means that there must
be a finite number of possible representations, but that's really an
implementation detail. I suppose you could figure out the
representation differences by fiddling with ctypes (in C I'd just use
a union), but that's really all.
ChrisA
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Re: Eval of expr with 'or' and 'and' within
On 15Jun2013 01:34, Steven D'Aprano
wrote:
| On Sat, 15 Jun 2013 00:09:31 +0100, Nobody wrote:
|
| > On Sat, 15 Jun 2013 03:56:28 +1000, Chris Angelico wrote:
| >> With a few random oddities:
| > bool(float("nan"))
| >> True
| >> I somehow expected NaN to be false. Maybe that's just my expectations
| >> that are wrong, though.
| >
| > In general, you should expect the behaviour of NaN to be the opposite of
| > what you expect.
|
| ... even taking that into account! *wink*
|
| Everyone is aware that there is more than one NAN, right?
I was not. Interesting.
| If my
| calculations are correct, there are 9007199254740992 distinct float NANs
| in Python (although there is no direct way of distinguishing them).
Wouldn't id() do it? At least in terms of telling them apart?
I gather they're not inspectable in Python?
Cheers,
--
Cameron Simpson
2 strokes are quicker than 4.
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Re: Eval of expr with 'or' and 'and' within
On Sat, 15 Jun 2013 00:09:31 +0100, Nobody wrote:
> On Sat, 15 Jun 2013 03:56:28 +1000, Chris Angelico wrote:
>
>> With a few random oddities:
>>
> bool(float("nan"))
>> True
>>
>> I somehow expected NaN to be false. Maybe that's just my expectations
>> that are wrong, though.
>
> In general, you should expect the behaviour of NaN to be the opposite of
> what you expect.
... even taking that into account! *wink*
Everyone is aware that there is more than one NAN, right? If my
calculations are correct, there are 9007199254740992 distinct float NANs
in Python (although there is no direct way of distinguishing them). Half
have the sign bit set, half do not; half are quiet NANs and half are
signalling NANs. It would be too easy if "sign bit set" meant signalling,
so in fact there are four equal-numbered groups of NANs, 2251799813685248
each of:
+quiet
+signalling
-quiet
-signalling
where the - sign should be interpreted as "sign bit is set" rather than
"negative", and + sign as "sign bit not set".
They differ according to their bit-pattern, or payload. Some systems
actually give standard meanings to different bit patterns, e.g. in the
old SANE (Standard Apple Numerics Environment) system, different NANs
were produced according to different kinds of errors, e.g:
Payload Description
=== ==
1Invalid sqrt
2Invalid addition, e.g. +INF + -INF
3Invalid division, e.g. 0/0
17 Convert invalid string
21 Attempt to create NAN with 0 as payload
etc.
(Assigning meaning to the payload is optional, according to the IEEE 754
standard, if I recall correctly.)
--
Steven
--
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Re: Eval of expr with 'or' and 'and' within
On 14Jun2013 12:50, Nikos as SuperHost Support wrote:
| I started another thread because the last one was !@#$'ed up by
| irrelevant replies and was difficult to jeep track.
|
| >>> name="abcd"
| >>> month="efgh"
| >>> year="ijkl"
|
| >>> print(name or month or year)
| abcd
|
| Can understand that, it takes the first string out of the 3 strings
| that has a truthy value.
|
| >>> print("k" in (name and month and year))
| True
|
| No clue. since the expression in parenthesis returns 'abcd' how can
| 'k' contained within 'abcd' ?
Did you print the result of "name and month and year"? It is the
_last_ value (if true at all). You used "or" in the first example
and "and" in the second.
| >>> print(name and month and year)
| ijkl
|
| Seems here is returning the last string out of 3 strings, but have
| no clue why Python doing this.
To evaluate an "and" it must test all of them to be true, and it
keeps the last value tested. (Or False, of course, if they are not
all true, in which case Python stops testing at the first False).
But for what you are doing, "and" and "or" are not good operations.
Something like:
"k" in (name+month+year)
or
"k" in name or "k" in month or "k" in year
is a more direct and accurate way to write what I imagine you're trying to do.
Cheers,
--
Cameron Simpson
No one is completely worthless... they can always serve as a bad example.
--
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Re: Eval of expr with 'or' and 'and' within
On 15/06/2013 00:06, Nobody wrote: On Fri, 14 Jun 2013 16:49:11 +, Steven D'Aprano wrote: Unlike Javascript though, Python's idea of truthy and falsey is actually quite consistent: Beyond that, if a user-defined type implements a __nonzero__() method then it determines whether an instance is true or false. If it implements a __len__() method, then an instance is true if it has a non-zero length. It's __nonzero__ in Python 2, __bool__ in Python 3. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Sat, Jun 15, 2013 at 5:33 AM, Grant Edwards wrote:
> On 2013-06-14, Chris Angelico wrote:
>> On Sat, Jun 15, 2013 at 3:49 AM, MRAB wrote:
>>> The general rule is that an object is true-ish unless it's false-ish
>>> (there are fewer false-ish objects than true-ish objects, e.g. zero vs
>>> non-zero int).
>>
>> With a few random oddities:
>>
> bool(float("nan"))
>> True
>>
>> I somehow expected NaN to be false. Maybe that's just my expectations
>> that are wrong, though.
>
> If you work with floating point long enough you realize that most of
> your expectations are wrong. Sometimes. Eventually.
NaN is like NULL in SQL. It's a weird beast. Just when you think
you've pinned it down, it slips out from under your taxonomic
classifications and thumbs its nose at you...
I think they're cousins to the platypus.
http://tvtropes.org/pmwiki/pmwiki.php/Main/EverythingsBetterWithPlatypi
ChrisA
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Re: Eval of expr with 'or' and 'and' within
On Sat, 15 Jun 2013 03:56:28 +1000, Chris Angelico wrote:
> With a few random oddities:
>
bool(float("nan"))
> True
>
> I somehow expected NaN to be false. Maybe that's just my expectations
> that are wrong, though.
In general, you should expect the behaviour of NaN to be the opposite of
what you expect.
--
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Re: Eval of expr with 'or' and 'and' within
On Fri, 14 Jun 2013 16:49:11 +, Steven D'Aprano wrote: > Unlike Javascript though, Python's idea of truthy and falsey is actually > quite consistent: Beyond that, if a user-defined type implements a __nonzero__() method then it determines whether an instance is true or false. If it implements a __len__() method, then an instance is true if it has a non-zero length. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Fri, 14 Jun 2013 19:30:27 +, Grant Edwards wrote: > 2. Returning one the objects that result from the evaluation of the > operands instead of returning True or False. > > This is what seems to be confusing him. This is much less common > than short-circuit evaluation. FWIW, Lisp also does this. But Lisp is slightly simpler as the only false value is "nil", while everything else is true (including integer zero). Although Python's any() and all() (which are closer to Lisp's "and" and "or" insofar as they all work with any number of values, including zero) always return True or False rather than the final value. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 2013-06-14, Nick the Gr33k wrote: > On 14/6/2013 7:47 , Benjamin Kaplan wrote: > In an "and" clause, >> python returns the first false value or the last value, because that >> will evaluate to the correct Boolean value. In an "or" clause, python >> returns the first true value or the last value. When Python finally got >> a Boolean type, no one wanted to break backwards compatibility for this. > > > This is exactly what i dont understand and thats why i keep asking and > people call me an idiot. I just dont understand why it behaves like that. > > Why return first or last value? There are cases where it's useful not only to know wether an expression was True or False, but _why_ it was true or false. > because that will evaluate to the correct Boolean value Yes. All values in Pyton have a "truthness" and you can be guaranteed that if A or B or C: will behave exactly the same way whether the "or" operator returns True or it returns one of it's operands. -- Grant Edwards grant.b.edwardsYow! Where do your SOCKS at go when you lose them in gmail.comth' WASHER? -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 2013-06-14, Chris Angelico wrote:
> On Sat, Jun 15, 2013 at 3:49 AM, MRAB wrote:
>> The general rule is that an object is true-ish unless it's false-ish
>> (there are fewer false-ish objects than true-ish objects, e.g. zero vs
>> non-zero int).
>
> With a few random oddities:
>
bool(float("nan"))
> True
>
> I somehow expected NaN to be false. Maybe that's just my expectations
> that are wrong, though.
If you work with floating point long enough you realize that most of
your expectations are wrong. Sometimes. Eventually.
--
Grant Edwards grant.b.edwardsYow! My pants just went to
at high school in the Carlsbad
gmail.comCaverns!!!
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Re: Eval of expr with 'or' and 'and' within
On 2013-06-14, Nobody wrote: > On Fri, 14 Jun 2013 18:16:05 +0300, Nick the Gr33k wrote: > >> My question is why the expr (name and month and year) result in the >> value of the last variable whic is variable year? > > For much the same reason that an OR expression returns the first true > value. > > "or" and "and" only evaluate as many arguments are required in order to > determine the correct result (aka "short-circuit evaluation"). If the > first argument of "or" is true, or the first argument of "and" is false, > the second argument isn't evaluated (this is important if evaluation can > have side effects). There are two completely orthogonal concepts here: 1. Short-circuit evaluation. Many languages do this. AFAICT, this isn't what he's asking about, but this is what people keep explaining. 2. Returning one the objects that result from the evaluation of the operands instead of returning True or False. This is what seems to be confusing him. This is much less common than short-circuit evaluation. C does short-circuit evaluation of && and || operators, but the result is always 1 or 0 (true of false). Instead of always returning True or False (which could be done and still preserver short-circuit evaluation), Python returns one of the operands or False. If you also have 1. there are cases where the value returned by the "or" operator is useful apart from it's "truthyness" value. There may be cases where the result returned by the "and" operator is useful apart from it's truthyness, but that seems to be less common. Taking advantage of that fact can lead to some hard-to-read code, so it's often discouraged as being too clever. It's important to note that these are somewhat orthogonal: You can have #1 and #2 (like Python). You can have #1 without #2 (like C). You can have #2 without #1 You can have neither #1 or #2 But again, #2 is more useful if you also have #1. -- Grant Edwards grant.b.edwardsYow! ... I have read the at INSTRUCTIONS ... gmail.com -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
Nick the Gr33k writes: > Why return first or last value? > > because that will evaluate to the correct Boolean value That value will either behave exactly the same as the Boolean value you call correct, or else it will be more useful. That is, most of the time it doesn't matter, and when it matters, Python's way is better. You can turn any expression E into a strictly Boolean value by writing bool(E) instead. Often E is already guaranteed to be a Boolean and the whole question does not arise in the first place. This doesn't prevent you from writing your program. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 14 June 2013 19:37, rusi wrote: > 2. The recent responses from Robert Kern are in my view the ideal. In > summary it runs thus: > Stupid question no. 6457 from Nikos: ... > Robert : Look this up > Nikos: I dont understand > Robert: explains > Nikos: I DONTU NDERSTND > Robert: explains (repeated as often as needed) > > When (if) finally Nikos actually reads the link and asks a more > specific/intelligent question, can be replaced by more specific > +1 Please, everyone else, just do this. This will calm the beast, I assure you, at least somewhat. If you're worried about not being "helpful", don't - this is as much help as ever. It's just it takes less time, and a *lot* less space. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Jun 14, 11:03 pm, Antoon Pardon wrote: > Op 14-06-13 18:09, Steven D'Aprano schreef: > > > > > > > > > > > On Fri, 14 Jun 2013 11:14:16 +0100, Robert Kern wrote: > > >> On 2013-06-14 10:50, Nick the Gr33k wrote: > > [snip question] > >>> This is all iw ant to know. > > >> This is all you need to read: > > >> http://docs.python.org/2/reference/expressions.html#boolean- > > operations > > > Thank you Robert for contributing a helpful response. > > > To everyone else... I know that Nikos' posts are draining. Sometimes he > > brings me to the brink of despair too. But if you aren't part of the > > solution, you are part of the problem: writing short-tempered, insulting > > posts after short-tempered, insulting post doesn't teach him, it just > > adds to EVERYBODY'S frustration with this never-ending serious of threads. > > That depends on your view. Maybe there are people who view you as part > of the problem because you keep giving Nikos hope other people will > solve his problems without him having to do anything significant himself. > > > Please keep the snarky comments offlist. Contribute something helpful if > > you like, mute the thread or killfile Nikos if you must, but adding to > > the volume of unproductive junk doesn't do anyone any favour. > > In my opinion the only really helpful thing would be to ignore nikos. He > has been given plenty or links, to read through in order to better his > understanding and i think it is about time people made it clear they > would no longer spoon feed him. Since that is probably not going to > happen and you will probably continue contributing to the annoyance > of other list members by continuing "helping" nikos, I don'y see why > other can't continue to the annoyance in a away that they themselves may > find somewhat amusing. > > -- > Antoon Pardon. Thanks Antoon. You are echoing the sentiments of probably hundreds of list subscribers. That said I would like to add a few points. 1. Beginning teachers (and young parents) are taught that one never should say: "You are a bad boy!" but rather "THIS behavior wont do" That nikos behavior is unacceptable has been reiterated by dozens of members in 100s of posts. That does not mean Nikos the person is unacceptable. We should always give him the room to change if/when he chooses to. 2. The recent responses from Robert Kern are in my view the ideal. In summary it runs thus: Stupid question no. 6457 from Nikos: ... Robert : Look this up Nikos: I dont understand Robert: explains Nikos: I DONTU NDERSTND Robert: explains (repeated as often as needed) When (if) finally Nikos actually reads the link and asks a more specific/intelligent question, can be replaced by more specific -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
Op 14-06-13 18:09, Steven D'Aprano schreef: On Fri, 14 Jun 2013 11:14:16 +0100, Robert Kern wrote: On 2013-06-14 10:50, Nick the Gr33k wrote: [snip question] This is all iw ant to know. This is all you need to read: http://docs.python.org/2/reference/expressions.html#boolean- operations Thank you Robert for contributing a helpful response. To everyone else... I know that Nikos' posts are draining. Sometimes he brings me to the brink of despair too. But if you aren't part of the solution, you are part of the problem: writing short-tempered, insulting posts after short-tempered, insulting post doesn't teach him, it just adds to EVERYBODY'S frustration with this never-ending serious of threads. That depends on your view. Maybe there are people who view you as part of the problem because you keep giving Nikos hope other people will solve his problems without him having to do anything significant himself. Please keep the snarky comments offlist. Contribute something helpful if you like, mute the thread or killfile Nikos if you must, but adding to the volume of unproductive junk doesn't do anyone any favour. In my opinion the only really helpful thing would be to ignore nikos. He has been given plenty or links, to read through in order to better his understanding and i think it is about time people made it clear they would no longer spoon feed him. Since that is probably not going to happen and you will probably continue contributing to the annoyance of other list members by continuing "helping" nikos, I don'y see why other can't continue to the annoyance in a away that they themselves may find somewhat amusing. -- Antoon Pardon. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Sat, Jun 15, 2013 at 3:49 AM, MRAB wrote:
> The general rule is that an object is true-ish unless it's false-ish
> (there are fewer false-ish objects than true-ish objects, e.g. zero vs
> non-zero int).
With a few random oddities:
>>> bool(float("nan"))
True
I somehow expected NaN to be false. Maybe that's just my expectations
that are wrong, though.
ChrisA
--
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Re: Eval of expr with 'or' and 'and' within
On 14/06/2013 18:28, Michael Torrie wrote: On 06/14/2013 10:49 AM, Steven D'Aprano wrote: Correct. In Python, all boolean expressions are duck-typed: they aren't restricted to True and False, but to any "true-ish" and "false-ish" value, or as the Javascript people call them, truthy and falsey values. There are a couple of anomalies -- the timestamp representing midnight is falsey, because it is implemented as a zero number of seconds; also exhausted iterators and generators ought to be considered falsey, since they are empty, but because they don't know they are empty until called, they are actually treated as truthy. But otherwise, the model is very clean. Good explanation! Definitely enlightened me. Thank you. The general rule is that an object is true-ish unless it's false-ish (there are fewer false-ish objects than true-ish objects, e.g. zero vs non-zero int). -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 06/14/2013 10:49 AM, Steven D'Aprano wrote: > Correct. In Python, all boolean expressions are duck-typed: they aren't > restricted to True and False, but to any "true-ish" and "false-ish" > value, or as the Javascript people call them, truthy and falsey values. > > There are a couple of anomalies -- the timestamp representing midnight is > falsey, because it is implemented as a zero number of seconds; also > exhausted iterators and generators ought to be considered falsey, since > they are empty, but because they don't know they are empty until called, > they are actually treated as truthy. But otherwise, the model is very > clean. Good explanation! Definitely enlightened me. Thank you. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 2013-06-14 18:01, Nick the Gr33k wrote: On 14/6/2013 7:47 μμ, Benjamin Kaplan wrote: In an "and" clause, python returns the first false value or the last value, because that will evaluate to the correct Boolean value. In an "or" clause, python returns the first true value or the last value. When Python finally got a Boolean type, no one wanted to break backwards compatibility for this. This is exactly what i dont understand and thats why i keep asking and people call me an idiot. I just dont understand why it behaves like that. Why return first or last value? because that will evaluate to the correct Boolean value How do you mean? Please elaborate. Please read the link I gave. It explains why. http://docs.python.org/2/reference/expressions.html#boolean-operations -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 14/6/2013 7:47 μμ, Benjamin Kaplan wrote: In an "and" clause, python returns the first false value or the last value, because that will evaluate to the correct Boolean value. In an "or" clause, python returns the first true value or the last value. When Python finally got a Boolean type, no one wanted to break backwards compatibility for this. This is exactly what i dont understand and thats why i keep asking and people call me an idiot. I just dont understand why it behaves like that. Why return first or last value? because that will evaluate to the correct Boolean value How do you mean? Please elaborate. -- What is now proved was at first only imagined! -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Fri, 14 Jun 2013 10:29:25 -0600, Michael Torrie wrote: > On 06/14/2013 03:50 AM, Nick the Gr33k wrote: >> >>> print(name or month or year) >> abcd >> >>> print(name and month and year) >> ijkl > > Interesting. I'd have thought a boolean expression would return True or > False, not a string. Learn something new every day. Correct. In Python, all boolean expressions are duck-typed: they aren't restricted to True and False, but to any "true-ish" and "false-ish" value, or as the Javascript people call them, truthy and falsey values. Unlike Javascript though, Python's idea of truthy and falsey is actually quite consistent: Truthy: - True - any non-zero number - any non-empty string - any non-empty list/tuple/dict/set etc. - object() - anything else best considered as "something" Falsey: - False - zero - empty string - empty list/tuple/dict/set etc. - None - anything else best considered as "nothing" There are a couple of anomalies -- the timestamp representing midnight is falsey, because it is implemented as a zero number of seconds; also exhausted iterators and generators ought to be considered falsey, since they are empty, but because they don't know they are empty until called, they are actually treated as truthy. But otherwise, the model is very clean. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Jun 14, 2013 9:34 AM, "Michael Torrie" wrote: > > On 06/14/2013 03:50 AM, Nick the Gr33k wrote: > > >>> print(name or month or year) > > abcd > > >>> print(name and month and year) > > ijkl > > Interesting. I'd have thought a boolean expression would return True or > False, not a string. Learn something new every day. > > > Python didn't have a Boolean type for quite some time (2.2?). Until then, True and False were ints. Since every object had a Boolean value, you didn't need to turn the result into an integer. In an "and" clause, python returns the first false value or the last value, because that will evaluate to the correct Boolean value. In an "or" clause, python returns the first true value or the last value. When Python finally got a Boolean type, no one wanted to break backwards compatibility for this. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Fri, 14 Jun 2013 18:16:05 +0300, Nick the Gr33k wrote: > My question is why the expr (name and month and year) result in the > value of the last variable whic is variable year? For much the same reason that an OR expression returns the first true value. "or" and "and" only evaluate as many arguments are required in order to determine the correct result (aka "short-circuit evaluation"). If the first argument of "or" is true, or the first argument of "and" is false, the second argument isn't evaluated (this is important if evaluation can have side effects). The operators can be expressed as: True or X = True False or X = X False and X = False True and X = X Note that in the short-circuit case, the result has the same sense (true or false) as the first argument, while in the other case the result has the same sense as the second argument. Python implements these operators by returning the actual value which determined the result of the expression rather than simply True or False. If the result is known after evaluating the first argument, the first argument is returned. If it has to evaluate the second argument, the second argument is returned (by that point it has already forgotten the value of the first argument). -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 06/14/2013 03:50 AM, Nick the Gr33k wrote: > >>> print(name or month or year) > abcd > >>> print(name and month and year) > ijkl Interesting. I'd have thought a boolean expression would return True or False, not a string. Learn something new every day. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Jun 14, 9:09 pm, Steven D'Aprano wrote: > On Fri, 14 Jun 2013 11:14:16 +0100, Robert Kern wrote: > > On 2013-06-14 10:50, Nick the Gr33k wrote: > [snip question] > >> This is all iw ant to know. > > > This is all you need to read: > > > http://docs.python.org/2/reference/expressions.html#boolean- > > operations > > Thank you Robert for contributing a helpful response. > > To everyone else... I know that Nikos' posts are draining. Sometimes he > brings me to the brink of despair too. But if you aren't part of the > solution, you are part of the problem: writing short-tempered, insulting > posts after short-tempered, insulting post doesn't teach him, it just > adds to EVERYBODY'S frustration with this never-ending serious of threads. Notice your spelling of series Steven? >From which I suggest you seriously(!) consider my earlier suggestion that Nikos is an infectious disease. -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On Fri, 14 Jun 2013 11:14:16 +0100, Robert Kern wrote: > On 2013-06-14 10:50, Nick the Gr33k wrote: [snip question] >> This is all iw ant to know. > > This is all you need to read: > >http://docs.python.org/2/reference/expressions.html#boolean- operations Thank you Robert for contributing a helpful response. To everyone else... I know that Nikos' posts are draining. Sometimes he brings me to the brink of despair too. But if you aren't part of the solution, you are part of the problem: writing short-tempered, insulting posts after short-tempered, insulting post doesn't teach him, it just adds to EVERYBODY'S frustration with this never-ending serious of threads. Please keep the snarky comments offlist. Contribute something helpful if you like, mute the thread or killfile Nikos if you must, but adding to the volume of unproductive junk doesn't do anyone any favours. Thank you. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 14/6/2013 5:49 μμ, Grant Edwards wrote:
On 2013-06-14, Nick the Gr33k wrote:
I started another thread
no kidding.
because the last one was !@#$'ed up by irrelevant replies and was
difficult to jeep track.
name="abcd"
month="efgh"
year="ijkl"
print(name or month or year)
abcd
Can understand that, it takes the first string out of the 3 strings
that has a truthy value.
Yes, it does. That's the way the language is defined to work. If you
don't like it, pick a different language.
I can understand OR stops at the first value when it finds it truthy,
and doesn't care for the other expr parameters.
And that logical since we say:
(name or month or year)
is like saying:
print whatever of those 3 are True, at least 1 of them, if it doesn't
not find any though for some reason it defaults to the last variable of
the expression(another mystery for me)
the way i realize it is just that for this boolean evaluation of the
expression to result in True if any of the above vars holds a truthy value.
Nice try, moving me to another newsgroup list :)
print("k" in (name and month and year))
True
No clue. since the expression in parenthesis returns 'abcd'
No it doesn't. Try it:
Python 2.7.3 (default, Mar 20 2013, 14:16:24)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
name="abcd"
month="efgh"
year="ijkl"
"k" in (name and month and year)
True
(name and month and year)
'ijkl'
how can 'k' contained within 'abcd' ?
Um sorry was a typo.
I mean the expr eval results in the value of var year which is 'ijkl'.
"k" in (name and month and year)
> True
So yes it does.
My question is why the expr (name and month and year) result in the
value of the last variable whic is variable year?
I cannot understand AND.
Isn't it like saying that:
(name and month and year)
all of these variables have to have truthy values so for the boolaean
eval of the expr result in True?
--
What is now proved was at first only imagined!
--
http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 2013-06-14, Nick the Gr33k wrote:
> I started another thread
no kidding.
> because the last one was !@#$'ed up by irrelevant replies and was
> difficult to jeep track.
>
> >>> name="abcd"
> >>> month="efgh"
> >>> year="ijkl"
>
> >>> print(name or month or year)
> abcd
>
> Can understand that, it takes the first string out of the 3 strings
> that has a truthy value.
Yes, it does. That's the way the language is defined to work. If you
don't like it, pick a different language.
> >>> print("k" in (name and month and year))
> True
>
> No clue. since the expression in parenthesis returns 'abcd'
No it doesn't. Try it:
Python 2.7.3 (default, Mar 20 2013, 14:16:24)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> name="abcd"
>>> month="efgh"
>>> year="ijkl"
>>>
>>> "k" in (name and month and year)
True
>>> (name and month and year)
'ijkl'
>>>
> how can 'k' contained within 'abcd' ?
It doesn't
--
Grant Edwards grant.b.edwardsYow! Am I having fun yet?
at
gmail.com
--
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Re: Eval of expr with 'or' and 'and' within
On 2013-06-14 10:50, Nick the Gr33k wrote:
I started another thread because the last one was !@#$'ed up by irrelevant
replies and was difficult to jeep track.
>>> name="abcd"
>>> month="efgh"
>>> year="ijkl"
>>> print(name or month or year)
abcd
Can understand that, it takes the first string out of the 3 strings that has a
truthy value.
>>> print("k" in (name and month and year))
True
No clue. since the expression in parenthesis returns 'abcd' how can 'k'
contained within 'abcd' ?
>>> print(name and month and year)
ijkl
Seems here is returning the last string out of 3 strings, but have no clue why
Python doing this.
>>> print("k" in (name and month and year))
True
>>>
yes, since expression returns 'ijkl', then the in operator can detect the 'k'
character within the returned string.
This is all iw ant to know.
This is all you need to read:
http://docs.python.org/2/reference/expressions.html#boolean-operations
Note the difference between how "or" and "and" each short-circuit. That is why
the (name or month or year) returns the first truthy value while (name and month
and year) returns the last truthy value. When "or" finds the first truthy value,
it can stop looking since the whole expression must be truthy no matter what the
values are after it. "and" cannot stop looking until it finds a falsy value or
runs out of values to look at.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
--
http://mail.python.org/mailman/listinfo/python-list
Re: Eval of expr with 'or' and 'and' within
On 14 Jun 2013 10:59, "Nick the Gr33k" wrote:
>
> I started another thread because the last one was !@#$'ed up by
irrelevant replies and was difficult to jeep track.
>
> >>> name="abcd"
> >>> month="efgh"
> >>> year="ijkl"
>
> >>> print(name or month or year)
> abcd
>
> Can understand that, it takes the first string out of the 3 strings that
has a truthy value.
>
> >>> print("k" in (name and month and year))
> True
>
> No clue. since the expression in parenthesis returns 'abcd' how can 'k'
contained within 'abcd' ?
>
> >>> print(name and month and year)
> ijkl
>
> Seems here is returning the last string out of 3 strings, but have no
clue why Python doing this.
>
> >>> print("k" in (name and month and year))
> True
> >>>
>
> yes, since expression returns 'ijkl', then the in operator can detect the
'k' character within the returned string.
>
> This is all iw ant to know.
You have been explained with both words, links and examples and now you
dismiss the previous post because it was "difficult to jeep track"?
Anyway. Search for "short circuit logic". Short circuit logic is why python
does it that way.
--
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Eval of expr with 'or' and 'and' within
I started another thread because the last one was !@#$'ed up by
irrelevant replies and was difficult to jeep track.
>>> name="abcd"
>>> month="efgh"
>>> year="ijkl"
>>> print(name or month or year)
abcd
Can understand that, it takes the first string out of the 3 strings that
has a truthy value.
>>> print("k" in (name and month and year))
True
No clue. since the expression in parenthesis returns 'abcd' how can 'k'
contained within 'abcd' ?
>>> print(name and month and year)
ijkl
Seems here is returning the last string out of 3 strings, but have no
clue why Python doing this.
>>> print("k" in (name and month and year))
True
>>>
yes, since expression returns 'ijkl', then the in operator can detect
the 'k' character within the returned string.
This is all iw ant to know.
--
What is now proved was at first only imagined!
--
http://mail.python.org/mailman/listinfo/python-list
