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() or __mro__ works only on new style classes. In python 3, they work without any issues. But in python 2 those classes need to inherit from object.

This would perhaps show the order of resolution.

class A(object):    def dothis(self):        print('I am from A class')class B(A):    passclass C(object):    def dothis(self):        print('I am from C class')class D(B, C):    passd_instance= D()d_instance.dothis()print(D.mro())

and response would be

I am from A class[<class '__main__.D'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.C'>, <class 'object'>]

The rule is depth-first, which in this case would mean D, B, A, C.

Python normally uses a depth-first order when searching inheriting classes, but when two classes inherit from the same class, Python removes the first mention of that class from mro.