'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.'

def __init__(cls, name, bases, dct):

    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
