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MRO vs Legacy & NewStyleClasses

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avimehenwal edited this page Mar 12, 2016 · 2 revisions

MRO: Method Resolution Order

In languages that use multiple inheritance, the order in which base classes are searched when looking for a method is often called the Method Resolution Order or MRO. (In Python this also applies to other attributes.) For languages that support single inheritance only, the MRO is uninteresting; but when multiple inheritance comes into play, the choice of an MRO algorithm can be remarkably subtle. Python has known at least three different MRO algorithms: classic, Python 2.2 new-style, and Python 2.3 new-style (a.k.a. C3). Only the latter survives in Python 3.

Classic classes used a simple MRO scheme: when looking up a method, base classes were searched using a simple depth-first left-to-right scheme. The first matching object found during this search would be returned. For example, consider these classes:

The crucial difference between resolution order MRO 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.

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