easy question on parsing python: "is not None"
in [Python]
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From: Bruno Desthuilliers on 6 Aug 2010 04:32 Richard D. Moores a écrit : > On Thu, Aug 5, 2010 at 16:15, Rhodri James <rhodri(a)wildebst.demon.co.uk> wrote: >> On Thu, 05 Aug 2010 17:07:53 +0100, wheres pythonmonks >> <wherespythonmonks(a)gmail.com> wrote: > >> You're not testing for equivalence there, you're testing for identity. "is" >> and "is not" test whether the two objects concerned are (or are not) the >> same object. Two objects can have the same value, but be different objects. >> The interpreter can fool you by caching and reusing objects which have the >> same value when it happens to know about it, in particular for small >> integers, but this is just a happy accident of the implementation and in no >> way guaranteed by the language. For example: >> >>>>> "spam, eggs, chips and spam" is "spam, eggs, chips and spam" >> True >>>>> a = "spam, eggs, chips and spam" >>>>> b = "spam, eggs, chips and spam" >>>>> a is b >> False >>>>> a == b >> True >> > > I'm wondering if there isn't considerable predictability to that > "happy accident". There is, indeed, but that's really implementation details. > Note how 1 'word' is treated versus 2: > >>>> x = 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>>> y = 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>>> x is y > True >>>> x = 'alksjdhflkajshdflkajhdflkjahsdflkj hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>>> y = 'alksjdhflkajshdflkajhdflkjahsdflkj hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' >>>> x is y > False CPython caches strings that happen to be valid Python identifiers. But once again, this is an implementation-specific optimization.
From: Jean-Michel Pichavant on 6 Aug 2010 05:42 Steven D'Aprano wrote: > On Thu, 05 Aug 2010 12:07:53 -0400, wheres pythonmonks wrote: > > >> P.S. Sorry for the top-post -- is there a way to not do top posts from >> gmail? I haven't used usenet since tin. >> > > Er, surely you can just move the cursor before you start typing??? > > > > CTRL+END will bring the cursor at the end of the file (for most the the mail clients). JM
From: Roald de Vries on 6 Aug 2010 06:01 On Aug 6, 2010, at 9:25 AM, Bruno Desthuilliers wrote: > Roald de Vries a écrit : >> 'not None' first casts None to a bool, and then applies 'not', so >> 'x is not None' means 'x is True'. > > Obviously plain wrong : > > Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41) > [GCC 4.3.3] on linux2 > Type "help", "copyright", "credits" or "license" for more information. > >>> 42 is not None > True > >>> 42 is True > False > >>> I know. I'm sorry. I'm embarrassed. I will check my facts better before posting in the future. And I'm sorry. Cheers, Roald
From: Vito 'ZeD' De Tullio on 6 Aug 2010 07:12 Richard D. Moores wrote: > So there would be a different implementation for each operating > system? One for Windows, one for linux? Or one for Vista and one for > XP? I'm just trying to clarify what is meant by "implementation". there are dozillions of "implementation" of python: one for each release of CPython (2.0, 2.1.2, 2.6.1, 3.x.y...), multiplied for each OS multiplied for IronPython, Jython, Pypy... etc... (obviously the "implementation details" between, say, CPython 2.6.1 and 2.6.2 are really minor vs Pypy X.Y.Z and IronPython A.B.C) -- By ZeD
From: DG on 6 Aug 2010 08:28
On Aug 6, 2:32 am, Bruno Desthuilliers <bruno. 42.desthuilli...(a)websiteburo.invalid> wrote: > Richard D. Moores a écrit : > > > > > On Thu, Aug 5, 2010 at 16:15, Rhodri James <rho...(a)wildebst.demon.co.uk> wrote: > >> On Thu, 05 Aug 2010 17:07:53 +0100, wheres pythonmonks > >> <wherespythonmo...(a)gmail.com> wrote: > > >> You're not testing for equivalence there, you're testing for identity. "is" > >> and "is not" test whether the two objects concerned are (or are not) the > >> same object. Two objects can have the same value, but be different objects. > >> The interpreter can fool you by caching and reusing objects which have the > >> same value when it happens to know about it, in particular for small > >> integers, but this is just a happy accident of the implementation and in no > >> way guaranteed by the language. For example: > > >>>>> "spam, eggs, chips and spam" is "spam, eggs, chips and spam" > >> True > >>>>> a = "spam, eggs, chips and spam" > >>>>> b = "spam, eggs, chips and spam" > >>>>> a is b > >> False > >>>>> a == b > >> True > > > I'm wondering if there isn't considerable predictability to that > > "happy accident". > > There is, indeed, but that's really implementation details. > > > Note how 1 'word' is treated versus 2: > > >>>> x = 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' > >>>> y = 'alksjdhflkajshdflkajhdflkjahsdflkjshadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' > >>>> x is y > > True > >>>> x = 'alksjdhflkajshdflkajhdflkjahsdflkj hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' > >>>> y = 'alksjdhflkajshdflkajhdflkjahsdflkj hadflkjhsadlfkjhaslkdjfhslkadhflkjshdflkjshdflkjshdfk' > >>>> x is y > > False > > CPython caches strings that happen to be valid Python identifiers. But > once again, this is an implementation-specific optimization. I've always thought of it as you don't compare strings with "is", you *should* use == The reasoning is that you don't know if that string instance is the only one in memory. I've heard as an implementation detail, since strings are immutable, that Python will only create one actual instance and just assign new references to it (your first x is y example), but to compare equality it just makes sense to use "==", not to ask if it is the same object in memory. Plus, I believe the "==" operator will check if the variables point to the same object. Using is/is not with None works well, because I believe there will always only be 1 None object. |