Question

from collections import *
class C(object):
    def __iter__(self): pass
    def __contains__(self, i): pass
    def __len__(self): pass
    def __getitem__(self, i): pass
issubclass(C, Mapping) => False
[issubclass(C, cls) for cls in Mapping.__mro__] => [False, True, True, True, True]

i.e. C does implement Sized, Iterable and Container.

I would have expected that just as issubclass(C, Sized) checks for the presence of a __len__ method, issubclass(C, Mapping) would check for the presence of the three methods required by each immediate superclass?

Answer 1

collections.Mapping is a mix-in class that provides the methods __contains__(), keys(), items(), values(), get(), __eq__() and __ne__() if you provide definitions of the methods __len__(), __iter__() and __getitem__(). For this to work, though, you need to derive from Mapping.

If you don’t want to derive from Mapping, you can also define all of the above-mentioned methods yourself and use

Mapping.register(C)

to make issubclass(C, Mapping) true.