What does "mro()" do?

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.

mro() stands for Method Resolution Order. It returns a list of types the class is derived from, in the order they are searched for methods.

mro() and __mro__ work only on new style classes. In Python 3, they work without any issues. In Python 2, however, those classes need to inherit from object.