Mock base class (old style classes)

a.py

class A:
    def method(self):
        print "I do stuff..."
    def uniquemethod(self):
        print "I'm only in A"

b.py

from a import A

class B(A):
    def method(self):
        A.method(self)
        print "But I can do some other stuff too !"

use.py

from mock import patch

with patch('b.A') as p:
    from b import B
    def testing(self):
        print "outside"
    p.method.side_effect = testing
    test = B()
    test.method()