for http://stackoverflow.com/questions/2542157/

models.py

from django.db import models

class Top(models.Model):
    _counter = 0
    def __init__(self, arg):
        Top._counter += 1
        print "Top(%s)#__init__(<%x>, %s) called %d times" % (
            self.__class__.__name__, id(self), arg, Top._counter)


def newA(cls, *args, **kwargs):
    print "O HAI %sx%x.__new__(%s, %s, %s)" % (
        cls.__name__, id(cls), cls, args, kwargs)
    if cls is A and len(args) > 0:
        fav = args[0]
        
        newcls = clsmap.get(fav)
        if newcls:            
            return newcls.__new__(newcls, *args, **kwargs)
        else:
            print "PRETENDING TO BE ABSTRACT"
            return None # or raise?
    else:
        return super(A, cls).__new__(cls, *args, **kwargs)


class A(Top):
    pass

# THIS IS THE TRICK!     O.o
A.__new__ = staticmethod(newA)

class B(A):
    fav = 1

class C(A):
    fav = 2

# could build this automagically with some A.__subclasses__() stuff maybe.
clsmap = {
    B.fav: B,
    C.fav: C
    }

assert A(0) is None

b = A(1)
assert isinstance(b, B)

c = A(2)
assert isinstance(c, C)