`dataclasses`: Synthetic `__init__` method on dataclass has broken `ClassVar` annotation under some conditions.

Bug report

Bug description:

When using automatically-stringified annotations (from __future__ import annotations), typing_extensions.ClassVar is not treated the same as typing.ClassVar even though those two objects are identical. Specifically, the dataclasses.dataclass-produced __init__ method does not exclude typing_extensions.ClassVar-wrapped variables from its annotations, but does exclude typing.ClassVar-wrapped variables from its annotations. Among other problems, this causes typing.get_type_hints to choke.

Reproducer below:

from __future__ import annotations

import dataclasses as dcls
import inspect
import typing
import typing_extensions as typx


assert typing.ClassVar is typx.ClassVar


@dcls.dataclass
class Foo:

    y: int
    x: typing.ClassVar[ int ] = 1


print( "\n=== typing.ClassVar (ok) ===" )

print( 'anno', Foo.__init__.__annotations__ )
print( 'inspect', inspect.get_annotations( Foo.__init__, eval_str = True ) )
print( 'typing', typing.get_type_hints( Foo.__init__ ) )
print( 'typx', typx.get_type_hints( Foo.__init__ ) )


@dcls.dataclass
class Bar:

    y: int
    x: typx.ClassVar[ int ] = 1


print( "\n=== typing_extensions.ClassVar (breaks) ===" )

print( 'anno', Bar.__init__.__annotations__ )
print( 'inspect', inspect.get_annotations( Bar.__init__, eval_str = True ) )
print( 'typing', typing.get_type_hints( Bar.__init__ ) )
print( 'typx', typx.get_type_hints( Bar.__init__ ) )

This results in the following output:

=== typing.ClassVar (ok) ===
anno {'y': 'int', 'return': None}
inspect {'y': <class 'int'>, 'return': None}
typing {'y': <class 'int'>, 'return': <class 'NoneType'>}
typx {'y': <class 'int'>, 'return': <class 'NoneType'>}

=== typing_extensions.ClassVar (breaks) ===
anno {'y': 'int', 'x': 'typx.ClassVar[int]', 'return': None}
inspect {'y': <class 'int'>, 'x': typing.ClassVar[int], 'return': None}
Traceback (most recent call last):
  File "/home/me/src/somepkg/bugs/dcls-classvar-hints.py", line 38, in <module>
    print( 'typing', typing.get_type_hints( Bar.__init__ ) )
  File "/usr/lib/python3.10/typing.py", line 1871, in get_type_hints
    value = _eval_type(value, globalns, localns)
  File "/usr/lib/python3.10/typing.py", line 327, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "/usr/lib/python3.10/typing.py", line 693, in _evaluate
    type_ = _type_check(
  File "/usr/lib/python3.10/typing.py", line 167, in _type_check
    raise TypeError(f"{arg} is not valid as type argument")
TypeError: typing.ClassVar[int] is not valid as type argument

(Credit to @Daraan for helping me pinpoint the problem.)

This is reproducible on both Python 3.10 and 3.13.

Possibly related to https://github.com/python/cpython/issues/89687.

CPython versions tested on:

3.10, 3.13

Operating systems tested on:

No response

Linked PRs

• gh-134073
• gh-140541

reproducer から始め、dataclasses.dataclass が ClassVar アノテーションに対して init.annotations をどのように構築するかを、typing.ClassVar と typing_extensions.ClassVar を比較しながら追跡します。生成されたアノテーションから ClassVar フィールドが除外され、typing.get_type_hints が例外を発生させずに動作することを確認して、fix を検証します。issue には関連する PR gh-134073 と gh-140541 が記載されています。