In python why does super behave this way with multiple inheritance?

Question

So we have a code snippet below. I don't understand why it behave this way. Why does super(B, self).go() resolves to the go method of the C class?

class A(object):
    def go(self):
        print("go A go!")

class B(A):
    def go(self):
        super(B, self).go()
        print("go B go!")

class C(A):
    def go(self):
        super(C, self).go()
        print("go C go!")

class D(B, C):
    def go(self):
        super(D, self).go()
        print("go D go!")

d = D()
d.go()
# go A go!
# go C go!
# go B go!
# go D go!

Have you read the MRO document yet? python.org/download/releases/2.3/mro explains not just the what, but the why. — Charles Duffy
– Charles Duffy, Commented Feb 4, 2016 at 22:01

chepner · Accepted Answer · 2016-02-04 21:59:14Z

5

Despite its name, super does not necessarily refer to a superclass. super(B, self) refers to the class in self's MRO that follows B.

You can see the MRO for D with

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

This means that from d.go, super(D, self) refers to B. When B.go is called as a result, super(B, self) refers to C, not A. This is because self is still an instance of D, so it is D.__mro__ that dictates what gets called next, not the static superclass of B.

The most important thing to remember about super is that inside Foo.go, you do not know what class super(Foo, self) will refer to, because you do not know if self is an instance of Foo or a descendent of Foo.

edited Feb 4, 2016 at 21:59

answered Feb 4, 2016 at 21:53

chepner

538k77 gold badges594 silver badges746 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Hamish Over a year ago

A useful thing to note: MRO stands for "method resolution order" - literally the order in which super methods are executed. Another way to think about this is "what would you expect to happen". It seems obvious that A.go should not be called twice. So if it should be called only once, when should it be called. The answer is the MRO order.

Collectives™ on Stack Overflow

In python why does super behave this way with multiple inheritance?

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related