C3 MRO

Consider the following class hierarchy:

class GrandParent:
    pass

class Parent1(GrandParent):
    pass

class Parent2(GrandParent):
    pass

class Child(Parent1, Parent2):
    pass

Which order will be used to look up the Child.x() method? The naive approach is to recursively search through all parent classes which gives us Child, Parent1, GrandParent, Parent2. While many programming languages follow this method indeed, it doesn't quite make sense, because Parent2 is more specific than GrandParent and should be looked up first.

In order to fix that problem, Python uses C3 superclass linearization, the algorithm that always searches for a method in all child classes before looking up the parent one:

In : Child.__mro__
Out:
(__main__.Child,
 __main__.Parent1,
 __main__.Parent2,
 __main__.GrandParent,
 object)