meta.py

#!/usr/bin/python
class ItemMetaClass(type):
    ITEM_CLASSES_INDEX = {}

    def __new__(mcs, name, bases, dict):
        cls = type.__new__(mcs, name, bases, dict)
        if hasattr(cls, 'id'):
            mcs.ITEM_CLASSES_INDEX[cls.id] = cls
        return cls

class Item(object):
    __metaclass__ = ItemMetaClass

    def __new__(cls, id=None, *args, **kwargs):
        if id in ItemMetaClass.ITEM_CLASSES_INDEX:
            return ItemMetaClass.ITEM_CLASSES_INDEX[id](*args, **kwargs)
        else:
            return super(Item, cls).__new__(cls)

class Hache(Item):
    id = 12

class Foo(Item):
    id = 42

print Item(12)
print Item(42)