What does “mro()” do?

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Follow along…:

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

As long as we have single inheritance, __mro__ is just the tuple of: the class, its base, its base’s base, and so on up to object (only works for new-style classes of course).

Now, with multiple inheritance…:

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

…you also get the assurance that, in __mro__, no class is duplicated, and no class comes after its ancestors, save that classes that first enter at the same level of multiple inheritance (like B and C in this example) are in the __mro__ left to right.

Every attribute you get on a class’s instance, not just methods, is conceptually looked up along the __mro__, so, if more than one class among the ancestors defines that name, this tells you where the attribute will be found — in the first class in the __mro__ that defines that name.

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