factories.py

class Engine(object):
    def __init__(self, size):
        self.size = size

    def turn_on(self):
        print "vrooom"


class BiggerEngine(object):
    def __init__(self, size):
        self.size = size

    def turn_on(self):
        print "VRROOOOOMMMM"


class CarFactory(object):
    def __new__(self, engine_class):
        class MyCar(object):
            def __init__(self, engine_size):
                self.engine = engine_class(size=engine_size)

        return MyCar


car_class = CarFactory(engine_class=Engine)
print car_class
car = car_class(engine_size="v4")
print car
car.engine.turn_on()

car_class = CarFactory(engine_class=BiggerEngine)
print car_class
car = car_class(engine_size="super")
print car
car.engine.turn_on()