def cast(cls, self):
    class _:
        def __getattribute__(_, name):
            attr = getattr(cls, name)
            if callable(attr):
                def _(*args, **kargs):
                    return attr(self, *args, **kargs)
                return _
            else:
                return getattr(self, name)
    return _()

class A:
    def func(self):
        print('A')

class B(A):
    def func(self):
        print('B')

b = B()
b.func() # => B
cast(A, b.func() # => A