[New-bugs-announce] [issue46642] typing: tested TypeVar instance subclass TypeError is incidental

[Gregory Beauregard]

New submission from Gregory Beauregard <greg at 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

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46642>
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