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. > 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
