False positive: comparison-overlap with dataclasses.Field.default_factory and dataclasses.MISSING

[sirosen]

Bug Report

In code which is checking if a field has a default, a check to see if Field.default_factory is MISSING will be misreported as a comparison-overlap error if mypy --strict is used.

To Reproduce

In foo.py:

from dataclasses import MISSING, Field
from typing import Any

def is_required(x: Field[Any]) -> bool:
    return x.default is MISSING and x.default_factory is MISSING

Expected Behavior

This check should pass. One of the valid values for default_factory is MISSING.

Actual Behavior

With the above reproduction,

$ mypy --strict foo.py
foo.py:5: error: Non-overlapping identity check (left operand type: "_DefaultFactory[Any]", right operand type: "_MISSING_TYPE")
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.931 and master (0.940+dev.84b977876aa76bfebf5fe49d11ebd202e4a60a33)
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): None

[erictraut]

This looks like a bug in the typeshed stub dataclasses.pyi. The type annotations for Field.default and Field.default_factory should probably be unions that include the type _MISSING_TYPE.

[sirosen]

Good point! I didn't think to look at typeshed -- I thought mypy was doing something weird.
I'll put in a PR to update it. Assuming that's right and my patch is accepted, I'll close this when that's merged.

[hauntsaninja]

There isn't any action for mypy, so closing. Current plan is to use the literal trick in typeshed: python/typeshed#7127