is None or == None ?

mk mrkafk at
Fri Nov 6 15:32:52 CET 2009

Stefan Behnel wrote:
> mk, 06.11.2009 14:20:
>> Some claim that one should test for None using:
>> if x is None:
> Which is the correct and safe way of doing it.


>> ..but the standard equality which is theoretically safer works as well:
>> if x == None:
> Absolutely not safe, think of
>    class Test(object):
>        def __eq__(self, other):
>            return other == None
>    print Test() == None, Test() is None

Err, I don't want to sound daft, but what is wrong in this example? It 
should work as expected:

 >>> class Test(object):
...     def __eq__(self, other):
...         return other == None
 >>> Test() is None
 >>> Test() == None

My interpretation of 1st call is that it is correct: instance Test() is 
not None (in terms of identity), but it happens to have value equal to 
None (2nd call).

Or perhaps your example was supposed to show that I should test for 
identity with None, not for value with None?

That, however, opens a can of worms, sort of: whether one should compare 
Test() for identity with None or for value with None depends on what 
programmer meant at the moment.


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