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typing.TypeAlias is not in the list of allowed plain _SpecialForm typeforms #90813
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GBeauregard commented
on Feb 6, 2022 GBeauregardmannequinMannequinAuthortyping.TypeAlias is allowed to be bare, but it's not listed in the list of types in typing._type_check that are allowed to be bare. This means it's possible to reach the wrong error
TypeError: Plain typing.TypeAlias is not valid as type argumentat runtime.Examples offhand:
from typing import TypeAlias, get_type_hints
class A:
a: "TypeAlias" = int
get_type_hints(A)from typing import Annotated, TypeAlias b: Annotated[TypeAlias, ""] = int
There's likely more and/or more realistic ways to trigger the problem. Anything that triggers typing._type_check on typing.TypeAlias will give this error (TypeError: Plain typing.TypeAlias is not valid as type argument).
I will fix this by adding TypeAlias to the list of typing special forms allowed to be bare/plain. I intend to move these to their own named var to reduce the chance of types not getting added in the future.
Looks like the more likely use case would be PEP-563:
from __future__ import annotations from typing import TypeAlias, get_type_hints import typing class C: a: TypeAlias = int print(get_type_hints(C))
This prints
Traceback (most recent call last): File "C:\Users\gvanrossum\cpython\t.py", line 9, in <module> print(get_type_hints(C)) File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 1808, in get_type_hints value = _eval_type(value, base_globals, base_locals) File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 326, in _eval_type return t._evaluate(globalns, localns, recursive_guard) File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 690, in _evaluate type_ = _type_check( File "C:\Users\gvanrossum\AppData\Local\Programs\Python\Python310\lib\typing.py", line 171, in _type_check raise TypeError(f"Plain {arg} is not valid as type argument")
Closing since the issue has been fixed (I cannot reproduce it on 3.12 or main).
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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