typing.TypeAlias is not in the list of allowed plain _SpecialForm typeforms

[GBeauregard]

BPO	46655
Nosy	@gvanrossum, @JelleZijlstra, @Fidget-Spinner, @GBeauregard
PRs	bpo-46655: allow stringized TypeAlias with get_type_hints #31156 [3.10] bpo-46655: allow stringized TypeAlias with get_type_hints (GH-… #31175

^{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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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-02-06.07:30:06.388>
labels = ['type-bug', 'library']
title = 'typing.TypeAlias is not in the list of allowed plain _SpecialForm typeforms'
updated_at = <Date 2022-02-07.16:22:04.935>
user = 'https://github.com/GBeauregard'

bugs.python.org fields:

activity = <Date 2022-02-07.16:22:04.935>
actor = 'gvanrossum'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-02-06.07:30:06.388>
creator = 'GBeauregard'
dependencies = []
files = []
hgrepos = []
issue_num = 46655
keywords = ['patch']
message_count = 4.0
messages = ['412618', '412685', '412686', '412766']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'JelleZijlstra', 'kj', 'GBeauregard']
pr_nums = ['31156', '31175']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46655'
versions = []

[GBeauregard]

typing.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 argument at 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.

[gvanrossum]

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")

[gvanrossum]

New changeset 77b025b by Gregory Beauregard in branch 'main':
bpo-46655: allow stringized TypeAlias with get_type_hints (GH-31156)
77b025b

[gvanrossum]

New changeset e2eeffe by Gregory Beauregard in branch '3.10':
[3.10] bpo-46655: allow stringized TypeAlias with get_type_hints (GH-31156). (bpo-31175)
e2eeffe

[picnixz]

Closing since the issue has been fixed (I cannot reproduce it on 3.12 or main).