On Fri, Jan 23, 2009 at 7:28 PM, Gary Herron <gher...@islandtraining.com>wrote:
> Steven D'Aprano wrote: > > On Fri, 23 Jan 2009 14:58:34 -0500, Gerald Britton wrote: > > > > > >> Hi -- Some time ago I ran across a comment recommending using <var> is > >> None instead of <var> == None (also <var> is not None, etc.) > >> > > > > That entirely depends on whether you wish to test for something which > > *is* None or something with *equals* None. Those two things have > > different meanings. > > > > Actually, for None, those two things *are* the same. If something > *equals* None, it also *is* None. This is a consequence of the fact > that there is only ever one value of None anywhere in the system. Not if someone decided to be a PITA. >>> class A(object) : ... def __eq__(self, other) : ... return other is None ... >>> a = A() >>> a == None True >>> a is None False > > > I wonder, do newbies actually get the impression from somewhere that "is" > > is a synonym for "=="? > > > > Yes. Such questions pop up regularly, and are usually dealt with quickly. > > > > > > > > >> My own > >> testing indicates that the former beats the latter by about 30% on > >> average. Not a log for a single instruction but it can add up in large > >> projects. > >> > > > > If you have a "large" project where the time taken to do comparisons to > > None is a significant portion of the total time, I'd be very surprised. > > > > var is None is a micro-optimization, but that's not why we do it. We do > > it because usually the correct test is whether var *is* None and not > > merely equal to None. Any random object might happen to equal None > > (admittedly most objects don't), but only None is None. > > > > > You don't have that quite right. The only way something can *equal* > None is if it *is* None. > None is not a value an object can have, but rather it is a (singleton) > object that can be referenced. Setting something *equal* to None is > accomplished by making it refer to the single None object, at which > point it *is* None. > > Gary Herron > > > > > > > -- > http://mail.python.org/mailman/listinfo/python-list >
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