gistfile1.py

#!/usr/bin/env python

class Base(object):
    def foo(self):
        print("Base")
        return True

class Mixin(object):
    def foo(self):
        print("Mixin logs bar {0}".format(self.bar))
        return super(Mixin, self).foo()

class A(Mixin, Base):
    def foo(self):
        print("A")
        self.bar = 1
        return super(A, self).foo()

a = A()
a.foo()

Output is

A
Mixin logs bar 1
Base