Calling Super Class Method In Multiple Inheritance

I have the following code: class A: pass class B(A): def foo(self, a): if a: return 'B' return super(B, self).foo(a) class C: def foo(self

Solution 1:

super() searches the MRO for the next class that has the attribute; that A doesn't implement it doesn't matter as C is still considered.

For D, the MRO is D, B, A, C:

>>> D.__mro__
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.C'>, <class 'object'>)

so super().foo in D will find B.foo, and from B.foo, A is skipped and C.foo is found; you can test this yourself from the interactive interpreter:

>>> super(D, d).foo
<bound method B.foo of <__main__.D object at 0x1079edb38>>
>>> super(B, d).foo
<bound method C.foo of <__main__.D object at 0x1079edb38>>

This is what a Python implementation of the attribute search algorithm would look like:

def find_attribute(type_, obj, name):
    starttype = type(obj)
    mro = iter(starttype.__mro__)

    # skip past the start type in the MRO
    for tp in mro:
        if tp == type_:
            break

    # Search for the attribute on the remainder of the MRO
    for tp in mro:
        attrs = vars(tp)
        if name in attrs:
            res = attrs[name]
            # if it is a descriptor object, bind it
            descr = getattr(type(res), '__get__', None)
            if descr is not None:
                res = descr(
                    res,
                    None if obj is starttype else obj,
                    starttype)
            return res

where type_ is the first argument to super() (the class the method is defined on), obj is the instance (so type(d) here), and name is the attribute you are looking for.