class Bullet(object)
    acceleration = (0,0)

    def start(position, direction):
        pass

class Missile(Bullet):
    def start(position, direction, target):
        self.target = target
        super(Missile, self).start(position, direction)

    def seek(self):
        pass

    def _process(self, delta):
        self.acceleration = seek()
        # etc etc
        super(Missile, self)._process(delta)