Crudely summarized, method resolution order is how Python decides where to look for a method (or any other attribute) on a class that inherits from several classes. Python actually has two; a simple one for old style classes and a rather complicated one for new style classes.

(Technically, method resolution order applies even for single inheritance classes, it's just that they have a very boring one.)

With the old style class:

class Foo(A, B, C):
    pass

Python will look up Foo.foo by looking for foo on A and all of its ancestors, then B and all its ancestors, and finally C and all its ancestors; that is, the method resolution order is left to right, depth first.

This order is nice and simple but blows up if A and B have a common ancestor with behavior that B wants to override, which is why new style classes need a different scheme. (All new style classes are ultimately rooted at object , which defines various default actions for things like getting and setting attributes.)

The complete description of the method resolution order for new style classes is somewhat complicated . For simple class structures, it can be summarized as left to right and depth first but common ancestor classes are only checked after all of their children have been checked . Thus, with new style classes:

class A1(object):       pass
class A2(object):       pass
class A3(object):       pass
class B2(A2):           pass
class B3(A3):           pass
class C3(B3):           pass
class Foo(C3, A1, B2):  pass

The method resolution order for Foo.foo is Foo, C3, B3, A3, A1, B2, A2, and then object ; as the common ancestor, object is checked only after all of its children have been. Note that the MRO can vary drastically between a parent and a child class; C3's MRO is just C3, B3, A3, and object .

In case of doubt or curiosity you can find the MRO of any new style class in its __mro__ attribute. Normally you don't need to care about its value and should use the super() builtin if you need to find the next class up in the context of a particular object or class.

(This is the kind of entry I write partly to make sure I have all this straight in my own head.)