[Python-Dev] NotImplemented comparisons

[Georg Brandl]

Terry Reedy schrieb:
> "Facundo Batista" <facundobatista at gmail.com> wrote in message 
> news:e04bdf310708021111g2870662bo5c6fdb3c1c68a9c2 at mail.gmail.com...
> | >>> class C(object):
> | ...     def __cmp__(self, other):
> | ...         return NotImplemented
> | ...
> 
> Given that you 'should' return an int, doing elsewise has undefined 
> results.

Returning anything other than an int or NotImplemented raises an exception.
NotImplemented seems to be special cased so that the other object's
__cmp__ can be tried too.

> | >>> c = C()
> | >>> print c < None
> 
> I presume that this translates into c.__compare(None) <  0 which becomes 
> NotImplemented < 0.  The result of that is undefined and interpreter 
> dependent.

No, it becomes id(c) < id(None). See half_compare in Objects/typeobject.c.


> This is still NotImplemented < 0 versus NotImplemented < None.  As I 
> understand, such nonsense comparisions will raise exceptions in 3.0.

Yes, fortunately.

Georg

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