Comment by cauthon

> It essentially allows the user to check if a class implements an interface, without explicitly inheriting ABC or Protocol.

Protocols don't need to be explicit superclasses for compile time checks, or for runtime checks if they opt-in with @runtime_checkable, but Protocols are also much newer than __subclass_hook__.

I don't think so. I think the other code should just stop using isinstance checks and switch to some custom function. I personally think isinstance checks benefit from having its behavior simpler and less dynamic.

> check if a class implements an interface, without explicitly inheriting ABC or Protocol

This really doesn't sound like a feature that belongs in the language. Go do something custom if you really want it.

But the moment you use a third party library, you cannot use it “responsibly” because that library, too, might use it “responsibly”, and then, you can easily get spooky interaction at a distance, with bugs that are hard or even impossible to fix.

A good example being stuff like isinstance(x, Iterable) and friends. Figuring out if something is iterable is a bit of a palaver otherwise.