Python MRO

MRO, Method Resolution Order

The MRO algorithm

1. List the classes that an instance inherits from using the classic class’s  DFLR(Depth First, Left to Right) lookup rule, and include a class multiple times if it’s visited more than once.
2. Scan the resulting list for duplicate classes, removing all but the last occurrence of duplicates in the list.

Tracing the MRO

>>> class A(object): attr = 1

>>> class B(A): pass

>>> class C(A): attr = 2

>>> class D(B, C): pass

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

>>> class A(object): attr = 1

>>> class B(A): pass

>>> class C(object): attr = 2

>>> class D(B, C): pass

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