Access method of second base class with super()

Question

Please explain to me the following. If I execute this:

class Base1:
    def foo(self):
        print('in Base1.foo')

b1 = Base1()
b1.foo()


class Base2:
    def foo(self):
        print('in Base2.foo')

b2 = Base2()
b2.foo()

class Child1(Base1, Base2):
    def foo(self):
        super(Child1,self).foo()

c1 =  Child1()
c1.foo()

class Child2(Base1, Base2):
    def foo(self):
        super(Base1,self).foo()

c2 =  Child2()
c2.foo()

I get this:

in Base1.foo
in Base2.foo
in Base1.foo
in Base2.foo

I understand the first three lines of the output. But why do I have to give the name of the first base class to super() in order to get the method of the second base class?

Answer 1

You are mucking with the Method Resolution Order , or MRO . Python class inheritance hierarchies are linearised , given a specific order, according to a system called the C3 linearisation .

super() uses that order to find the next attribute (including methods) in that ordering. Given a current instance, and a class, it'll search the order for an attribute past the given class. For both Child* classes, the MRO lists the Child* class first, followed by Base1 and Base2 . In your last example you told super() to go looking in that MRO starting from the next class after Base1 , so only Base2.foo() remains to be found.

You can ask any class for its MRO by calling the class.mro() method :

>>> Child2.mro()
[<class '__main__.Child2'>, <class '__main__.Base1'>, <class '__main__.Base2'>, <class 'object'>]

super() can be used outside methods too; you can then easily see what happens when you pick different starting points for the search:

>>> super(Child2, Child2()).foo
<bound method Base1.foo of <__main__.Child2 object at 0x10f40ff98>>
>>> super(Base1, Child2()).foo
<bound method Base2.foo of <__main__.Child2 object at 0x10f40ffd0>>
>>> super(Base2, Child2()).foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'super' object has no attribute 'foo'