def __init__(cls, name, bases, dct):
    """Initializes a class that belongs to a polymorphic hierarchy.

    This method configures a few built-in attributes of polymorphic
    models:

      __root_class__: If the new class is a root class, __root_class__ is set to
        itself so that it subclasses can quickly know what the root of
        their hierarchy is and what kind they are stored in.
      __class_hierarchy__: List of classes describing the new model's place
        in the class hierarchy in reverse MRO order.  The first element is
        always the root class while the last element is always the new class.

        MRO documentation: http://www.python.org/download/releases/2.3/mro/

        For example:
          class Foo(PolymorphicClass): ...

          class Bar(Foo): ...

          class Baz(Bar): ...

          Foo.__class_hierarchy__ == [Foo]
          Bar.__class_hierarchy__ == [Foo, Bar]
          Baz.__class_hierarchy__ == [Foo, Bar, Baz]

    Unless the class is a root class or PolyModel itself, it is not
    inserted in to the kind-map like other models.  However, all polymorphic
    classes, are inserted in to the class-map which maps the class-key to
    implementation.  This class key is consulted using the polymorphic instances
    discriminator (the 'class' property of the entity) when loading from the
    datastore.
    """
    if name == 'PolyModel':
      super(PolymorphicClass, cls).__init__(name, bases, dct, map_kind=False)
      return

    elif PolyModel in bases:
      if getattr(cls, '__class_hierarchy__', None):
        raise db.ConfigurationError(('%s cannot derive from PolyModel as '
            '__class_hierarchy__ is already defined.') % cls.__name__)
      cls.__class_hierarchy__ = [cls]
      cls.__root_class__ = cls
      super(PolymorphicClass, cls).__init__(name, bases, dct)
    else:
      super(PolymorphicClass, cls).__init__(name, bases, dct, map_kind=False)

      cls.__class_hierarchy__ = [c for c in reversed(cls.mro())
          if issubclass(c, PolyModel) and c != PolyModel]

      if cls.__class_hierarchy__[0] != cls.__root_class__:
        raise db.ConfigurationError(
            '%s cannot be derived from both root classes %s and %s' %
            (cls.__name__,
            cls.__class_hierarchy__[0].__name__,
            cls.__root_class__.__name__))

    _class_map[cls.class_key()] = cls
dataset/ETHPy150Open CollabQ/CollabQ/.google_appengine/google/appengine/ext/db/polymodel.py/PolymorphicClass.__init__/original