New submission from Ivan Levkivskyi <levkivs...@gmail.com>:
The PEP 544 specifies that: A protocol can be used as a second argument in isinstance() and issubclass() only if it is explicitly opt-in by @runtime_checkable decorator. It is not specified exactly whether this should be enforced by static type checkers or at runtime. Currently it is enforced in both cases: mypy flags this as error, and a TypeError is raised at runtime. There is however a problem with current runtime implementation: abc and functools modules may call issubclass() on non-runtime checkable protocols if they appear as explicit superclasses. This is currently solved by a sys._getframe() hack in typing module. The problem is that current solution is incomplete. For example, the TypeError is not raised in the case of non-method protocols: >>> class P(Protocol): ... x: int ... >>> >>> class C: ... ... >>> isinstance(C(), P) False # should be TypeError I tried to fix it this morning but after an hour of attempts to tweak the existing hack I gave up. It looks like there are only two reasonable solutions: * Don't enforce @typing.runtime_checkable at runtime, make it a type-checker-only feature (like @typing.final). * Add a little helper to abc module that would skip classes in MRO for which C._is_protocol is True but C._is_runtime_protocol is False. Any thoughts/preferences? ---------- components: Library (Lib) messages: 357400 nosy: gvanrossum, levkivskyi priority: normal severity: normal status: open title: Troubles with @runtime_checkable protocols type: behavior versions: Python 3.8, Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue38908> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
