Auto inherit inner classes

inner_inherit.py

class M(type):
    def __new__(mcs, name, bases, attrs):
        if bases:
            b = attrs.pop('B', None)
            if b:
                bs = tuple(s for s in (c.__dict__.get('B') for c in bases) if s)
                attrs['B'] = type('B', bs, b.__dict__)
        return super(M, mcs).__new__(mcs, name, bases, attrs)


class A(object):
    __metaclass__ = M

    class B:
        foo = 1
        bar = 2

        @classmethod
        def spam(cls):
            return cls.foo + cls.bar


class D(A):
    class B:
        baz = 3

        @classmethod
        def spam(cls):
            return super(D.B, cls).spam() + cls.baz


assert isinstance(A, M)
assert A.B.foo == 1
assert A.B.bar == 2
assert A.B.spam() == 3

assert isinstance(D, M)
assert issubclass(D.B, A.B)
assert D.B.foo == 1
assert D.B.bar == 2
assert D.B.baz == 3
assert D.B.spam() == 6