To hook this other object in, override make_object(added_class, name, **attrs) and maybe set_object(added_class, name, **attrs) (the default implementation of set_object just resets the attribute to whatever make_object returned).
Also see BoundFactory.
<bound class method BoundAttribute.__addtoclass__>
<bound class method BoundAttribute.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.