"Facundo Batista" <facundobatista@gmail.com> wrote in message news:e04bdf310708021111g2870662bo5c6fdb3c1c68a9c2@mail.gmail.com... | >>> class C(object): | ... def __cmp__(self, other): | ... return NotImplemented | ... Given that you 'should' return an int, doing elsewise has undefined results. | >>> 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. | >>> print NotImplemented < None As is this. There is no reason to expect the two comparisons (NotImplemented to 0 and None) to give the same or different results. | Does somebody know why is a difference here? Different interpreters, different arbitrary results. I believe checking the ids of the right objects (the type objects, I have read) would explain. | Furthermore, we can check that is a problem regarding __cmp__: | | >>> class C(object): | def __cmp__(self, other): | return NotImplemented | def m(self): | return NotImplemented | | >>> c = C() | >>> print c < None | True | >>> print c.m() < None | False This is still NotImplemented < 0 versus NotImplemented < None. As I understand, such nonsense comparisions will raise exceptions in 3.0. tjr