Calling super class method in multiple inheritance

Question

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, a):
        return 'C'

class D(B, C):
    def foo(self, a):
        return super().foo(a)

d = D()
print(d.foo(0))

When I call d.foo(0) based on MRO it first calls the foo method of B class and inside that, if the condition is wrong and it will return super(B, self).foo(0) but class A has no foo method and I expect this error:

AttributeError: 'super' object has no attribute 'foo'

but it returns 'C' from class C. Why?

That's the whole point of super. Think about it - if it were to call the parent directly, you wouldn't need super in the language at all, you would just call A directly here. — wim
– wim, Commented Jul 29, 2017 at 14:49

Martijn Pieters · Accepted Answer · 2017-07-29 14:59:49Z

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.

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