mypy generates `__match_args__` for dataclasses and `NamedTuple`s on Python <3.10

[AlexWaygood]

Bug Report

Not a big deal, but mypy appears to generate __match_args__ for dataclasses and NamedTuples even when running with --python-version set to 3.9:

from dataclasses import dataclass
from typing import NamedTuple

@dataclass
class One:
    bar: int

f: One
reveal_type(f.__match_args__)  # Revealed type is "Tuple[Literal['bar']]"

class Two(NamedTuple):
    baz: str

g: Two
reveal_type(g.__match_args__)  # Revealed type is "Tuple[Literal['baz']]"

Expected Behavior

Mypy should not autogenerate __match_args__ when running with --python-version set to <3.10, as pattern-matching is new in 3.10.

Your Environment

• Mypy version used: 0.950+dev.a33d2353a81d075850a487f3913720037f2473aa (compiled: no)
• Mypy command-line flags: --python-version 3.9
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10
• Operating system and version: Windows

[jhance]

This is probably a good issue for someone new to mypy.

[Ryanchou24]

I am new to mypy and I would like to work on this. I search the code above but was unable to find it. Can someone tells me where should I look at it?

[Ryanchou24]

I was wondering that if this solves the problem.

[AlexWaygood]

I was wondering that if this solves the problem.

I haven't looked at the code in depth, but that looks promising!

Pattern-matching support was added in #10191 — if you do a CTRL-F for "dataclasses" and "NamedTuple" in the files-changed tab for that PR, you should be able to find the bits of code that need to be changed 🙂

[Loriasun]

Hi, there I am wondering if I can work on this one?

[AlexWaygood]

@Loriasun, there's already an open PR for this one (#12503) — it would be better to wait until that has been reviewed before opening another PR for this issue 🙂