is None or == None ?

Terry Reedy tjreedy at
Sat Nov 7 21:31:02 CET 2009

Steven D'Aprano wrote:
> On Fri, 06 Nov 2009 16:51:18 +0100, Marco Mariani wrote:
>> Using "x is y" with integers
>> makes no sense and has no guaranteed behaviour AFAIK
> Of course it makes sense. `x is y` means *exactly the same thing* for 
> ints as it does with any other object: it tests for object identity. 
> That's all it does, and it does it perfectly.
> Python makes no promise whether x = 3; y = 3 will use the same object for 
> both x and y or not. That's an implementation detail. That's not a 
> problem with `is`, it is a problem with developers who make unjustified 
> assumptions.

Which is to say, it normally makes no sense to write 'm is n' for m, n ints.

The *exception* is when one is exploring implementation details, either 
to discover them or to test that they are as intended. So, last I 
looked, the test suite for ints makes such tests. If the implementation 
changes, the test should change also.

The problem comes when newbies use 'is' without realizing that they are 
doing black-box exploration of otherwise irrelevant internals. 
(White-box exploration would be reading the code, which makes it plain 
what is going on ;-).

Terry Jan Reedy

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