Puzzling OO design problem

[George Sakkis]

> Err, you might want to explain what these things do instead of an
> abstract description of how you are doing it.  It looks like you are
> using inheritance in the normal way _and_ you are using it to handle
> versioning of some kind (maybe stable interface releases? I don't
know).
>
> Let us know what parts need inheritance for, and what you have
> just been using a side effect of inheritance for as a convenience
> (versioning, I think).
>
> A more concrete example would be easier to comment on, if possible
> do a simple one (maybe just two classes with two versions each).
>
> -jackdied

I intentionally abstracted the problem to remove the irrelevant
details, but here's a more concrete (though still simplified) example.
I hope it is more clear now.

George

#=============================

def worldModelFactory(version):
    if version < 2: return WorldModel()
    else: return WorldModel_v2()

class WorldModel(object):

    def __init__(self):
        self.ourGoal = self.FieldObject(x=-50, y=0)
        self.theirGoal = self.FieldObject(x=+50, y=0)
        self.ball = self.MovableObject()
        self.teammates = [self.Player(i) for i in xrange(1,12)]
        self.opponents = [self.Player(i) for i in xrange(1,12)]

    class FieldObject(object):
        def __init__(self, id=None, x=0, y=0):
            self.id = id
            self._pos = (x,y)
        def position(self):
            '''Get or estimate the current position.'''
            return self._pos

    class MovableObject(FieldObject):
        def speed(self):
            '''Get or estimate the current speed.'''
            # [implementation snipped]

    class Player(MovableObject):
        def passBall(self,power,teammate):
            '''Pass the ball to the teammate.'''
            # [implementation snipped]


class WorldModel_v2(WorldModel):

    class FieldObject(WorldModel.FieldObject):
        '''New implementation of FieldObject.'''
        def position(self):
            # [new implementation snipped]
            pass

    class MovableObject(WorldModel.MovableObject, FieldObject):
        '''MovableObject didn't change since the previous version. The
           only reason for this class is to make
WorldModel_v2.FieldObject
           accessible to WorldModel_v2.Player.
        '''
        pass

    class Player(WorldModel.Player, MovableObject):
        '''New implementation of Player.'''
        def passBall(self,power,teammate):
            # WorldModel_v2.FieldObject.position() should be called
            myPosition = self.position()
            # [new implementation snipped]

#=============================