python3.10 interacts badly with typing_extensions.ClassVar

[graingert]

from typing import Callable, Optional, ClassVar

import typing_extensions
import attr


@attr.s(auto_attribs=True)
class Ham:
    ham: typing_extensions.ClassVar[int] = 0
    spam: int
    eggs: Optional[int] = attr.ib(init=False, default=None)

python3.10 foo.py
Traceback (most recent call last):
  File "/home/graingert/projects/twisted/foo.py", line 8, in <module>
    class Ham:
  File "/home/graingert/projects/twisted/.tox/py310-alldeps-nocov/lib/python3.10/site-packages/attr/_make.py", line 1292, in wrap
    builder = _ClassBuilder(
  File "/home/graingert/projects/twisted/.tox/py310-alldeps-nocov/lib/python3.10/site-packages/attr/_make.py", line 604, in __init__
    attrs, base_attrs, base_map = _transform_attrs(
  File "/home/graingert/projects/twisted/.tox/py310-alldeps-nocov/lib/python3.10/site-packages/attr/_make.py", line 518, in _transform_attrs
    raise ValueError(
ValueError: No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: Attribute(name='spam', default=NOTHING, validator=None, repr=True, eq=True, order=True, hash=None, init=True, metadata=mappingproxy({}), type='int', converter=None, kw_only=False, inherited=False, on_setattr=None)

[graingert]

it seems to work fine if I use from typing_extensions import ClassVar

from typing import Callable, Optional, ClassVar

from typing_extensions import ClassVar
import attr


@attr.s(auto_attribs=True)
class Ham:
    ham: ClassVar[int] = 0
    spam: int
    eggs: Optional[int] = attr.ib(init=False, default=None)

[euresti]

Yeah we just do a string comparison on typing.ClassVar or ClassVar. Can you use from typing import ClassVar? (I don't recall which supported versions of python include it these days)

[graingert]

@euresti here's the context:
https://github.com/twisted/twisted/pull/1559/files#diff-592f0a2a781d4b88035bcd58b8686ce792e80e17b7e93cb25bba26efab963250R958

[graingert]

I'm avoiding the from ... import to avoid clashing with twisted.internet.Protocol

[euresti]

Ah. Try

import typing

typing.ClassVar[...]

This should be available in python > 3.5.3.

If you need to support older pythons you can put it in quotes. i.e.

_log: "typing.ClassVar[Logger]" = Logger()

[euresti]

Note: attrs should probably support typing_extensions.ClassVar I'm just trying to give you a workaround.

[graingert]

Ah I should totally be using from typing import ClassVar I mistakenly thought it was new in 3.8