Hello everybody! In a personnal project I feel the need (or the desire) to implement something like this: assert isinstance(1, PositiveInteger) assert not isinstance(-1, PositiveInteger) So I began looking a lot in the abc module, and I end unp using an __instancehook__ special method wich is called by __instancechek__ in the corresponding metaclass, just like the __subclasshook__ special method called by __subclasscheck__. The core idea is something like this: class MyMeta(type): def __instancecheck__(cls, instance): return cls.__instancehook__(instance) class PositiveInteger(metaclass=MyMeta): @classmethod def __instancehook__(cls, instance): return isinstance(instance, int) and instance > 0 Of course, the real implemention is more detailed... What do you think about that ?