Infer type of `attrs.fields(type(attrs_instance))`

[injust]

Feature

mypy currently infers the type of attrs.fields(foo) as Any, where foo is an instance of an attrs class. Is it possible to have mypy infer the correct type?

With this setup:

import attrs
from attrs import define


@define
class Foo:
    bar: int


foo = Foo(1)

mypy infers the correct type for attrs.fields(Foo):

fields = attrs.fields(Foo)
reveal_type(fields)
reveal_type(fields.bar)
reveal_type(fields.not_bar)

"""
demo.py:13: note: Revealed type is "tuple[attr.Attribute[builtins.int], fallback=demo.Foo.__demo_Foo_AttrsAttributes__]"
demo.py:14: note: Revealed type is "attr.Attribute[builtins.int]"
demo.py:15: error: "__demo_Foo_AttrsAttributes__" has no attribute "not_bar"  [attr-defined]
demo.py:15: note: Revealed type is "Any"
Found 1 error in 1 file (checked 1 source file)
"""

but mypy infers attrs.fields(type(foo)) as Any:

fields = attrs.fields(type(foo))
reveal_type(fields)
reveal_type(fields.bar)
reveal_type(fields.not_bar)

"""
demo.py:13: note: Revealed type is "Any"
demo.py:14: note: Revealed type is "Any"
demo.py:15: note: Revealed type is "Any"
Success: no issues found in 1 source file
"""

Pitch

Passing the class to attrs.fields() is the documented usage.

If you pass type(attrs_instance) instead, mypy quietly treats the type as Any (even under strict mode), so it cannot catch attr-defined errors when you access a non-existent field.

ref python-attrs/attrs#1297

[injust]

@Tinche called out here:

There's probably a difference between Foo and type(foo) - the latter can be any arbitrary subclass of Foo.

But even if you use attrs.fields() in a method of an attrs class, mypy still infers attrs.fields(type(self)) and attrs.fields(cls) as Any.