Method Resolution Order (MRO) in new style Python classes

Stack Overflow link: http://stackoverflow.com/questions/1848474/method-resolution-order-mro-in-new-style-python-classes

The crucial difference between resolution order for legacy vs new-style classes comes when the same ancestor class occurs more than once in the "naive", depth-first approach -- e.g., consider a "diamond inheritance" case:

>>> class A: x = 'a'
... 
>>> class B(A): pass
... 
>>> class C(A): x = 'c'
... 
>>> class D(B, C): pass
... 
>>> D.x
'a'

here, legacy-style, the resolution order is D - B - A - C - A : so when looking up D.x, A is the first base in resolution order to solve it, thereby hiding the definition in C. While:

>>> class A(object): x = 'a'
... 
>>> class B(A): pass
... 
>>> class C(A): x = 'c'
... 
>>> class D(B, C): pass
... 
>>> D.x
'c'
>>>

here, new-style, the order is:

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

with A forced to come in resolution order only once and after all of its subclasses, so that overrides (i.e., C's override of member x) actually work sensibly.

It's one of the reasons that old-style classes should be avoided: multiple inheritance with "diamond-like" patterns just doesn't work sensibly with them, while it does with new-style.