<var> is None vs. <var> == None

[Gary Herron]

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

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