The factory will be reinvoked (and the attribute rebound) for every subclassing.
<bound class method BoundFactory.__addtoclass__>
<bound class method BoundFactory.singleton>
Abstract classes can override this to customize issubclass().
This is invoked early on by abc.ABCMeta.__subclasscheck__(). It should return True, False or NotImplemented. If it returns NotImplemented, the normal algorithm is used. Otherwise, it overrides the normal algorithm (and the outcome is cached).
See the source for more information.