Do not add kw_only dataclass fields to __match_args__

[sobolevn]

In runtime python does not add fields that have kw_only marker:

• https://github.com/python/cpython/blob/895d983b5c9716aaaab34d14d278084b9b6730d8/Lib/dataclasses.py#L1174-L1177
• https://github.com/python/cpython/blob/895d983b5c9716aaaab34d14d278084b9b6730d8/Lib/dataclasses.py#L411-L417

See:

>>> import dataclasses
>>> @dataclasses.dataclass(kw_only=True)
... class A:
...     a: int
... 
>>> print(A.__match_args__)
()

Closes #18863

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Looks good! Consider also submitting a documentation bug to python/cpython: now the docs fail to mention this rather important aspect (and even seem to suggest the opposite?).

match_args: If true (the default is True), the match_args tuple will be created from the list of parameters to the generated init() method (even if init() is not generated, see above).

kw_only: If true (the default value is False), then all fields will be marked as keyword-only. If a field is marked as keyword-only, then the only effect is that the init() parameter generated from a keyword-only field must be specified with a keyword when init() is called. There is no effect on any other aspect of dataclasses. See the parameter glossary entry for details. Also see the KW_ONLY section.

[sobolevn]

Consider also submitting a documentation bug to python/cpython

Agreed, working on it!

Thanks!