Inheritance and composition are isomorphic

gistfile1.py

class Parent(object):
    def __init__(self, x):
        self.x = x

    def frob(self):
        self.x += 1


class Child1(Parent):
    def __init__(self, x, y):
        self.y = y
        super(Child1, self).__init__(x)

    def frob(self):
        self.y += 1
        super(Child1, self).frob()


class Child2(object):
    def __init__(self, x, y):
        self.y = y
        self.parent = Parent(x)

    def frob(self):
        self.y += 1
        self.parent.frob()

    @property
    def x(self):
        return self.parent.x

There is a mechanical transformation from Child1 to Child2 and vice-versa.