New submission from Gregory Beauregard <g...@greg.red>:
https://github.com/python/cpython/blob/bf95ff91f2c1fc5a57190491f9ccdc63458b089e/Lib/test/test_typing.py#L227-L230 typing's testcases contain the following test to ensure instances of TypeVar cannot be subclassed: def test_cannot_subclass_vars(self): with self.assertRaises(TypeError): class V(TypeVar('T')): pass The reason this raises a TypeError is incidental and subject to behavior change, not because doing so is prohibited per se; what's happening is the class creation does the equivalent of type(TypeVar('T')(name, bases, namespace), but this calls TypeVar's __init__ function with these items as the TypeVar constraints. TypeVar runs typing._type_check on the type constraints passed to it, and the literals for the namespace/name do not pass the callable() check in typing._type_check, causing it to raise a TypeError. I find it dubious this is the behavior the testcase is intending to test and the error it gives is confusing I propose we add __mro_entries__ to the TypeVar class that only contains a raise of TypeError to properly handle this case I can write this patch ---------- components: Library (Lib) messages: 412544 nosy: AlexWaygood, GBeauregard, Jelle Zijlstra, gvanrossum, kj, sobolevn priority: normal severity: normal status: open title: typing: tested TypeVar instance subclass TypeError is incidental type: enhancement _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue46642> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
