gistfile1.py

class Additive(type):
    def __add__(self, x):
        return type("Additive", (self, x), {})

class A(object):
    __metaclass__ = Additive
    def foo(self):
        print "foo"

class B(object):
    def bar(self):
        print "bar"

ab = (A+B)()
ab.bar() => "bar"
ab.foo() => "foo"