Iterate through __slots__

[Gribouillis]

Mro is the way to go. Use

def all_slots(ob):
    if not hasattr(ob, 'mro'):
        ob = type(ob)
    for cls in reversed(ob.mro()):
        yield from cls.__dict__.get('__slots__', ())

and replace for attr in self.__slots__ with for attr in all_slots(self)

Edit: bugfix: changed getattr to __dict__.get to avoid repetition in case subclasses don't have slots.