Error when accessing dataclasses.Field.default_factory

[sloria]

Typechecking with mypy produces a spurious error when accessing default_factory of a dataclass field.

# t.py
import dataclasses


@dataclasses.dataclass
class Foo:
    bar: int


field = dataclasses.fields(Foo)[0]
assert field.default_factory == dataclasses.MISSING

mypy t.py

Output:

t.py:10: error: Attribute function "default_factory" with type "Callable[[], Any]" does not accept self argument

Using mypy==0.720 on Python 3.7.4.

[srittau]

This looks like a mypy problem to me. The following does not type check either:

from typing import Callable

class Foo:
    bar: Callable[[], None]

assert Foo().bar == 1

[sloria]

Ah, found it: python/mypy#6910 . Sorry for the noise.