AttrsInstance is only a Protocol in mypy

[euresti]

__init__.pyi defines AttrsInstance as follows:

class AttrsInstance(AttrsInstance_, Protocol):
    pass

So the following typechecks correctly:

import attr
from typing import Protocol

class Foo(attr.AttrsInstance, Protocol):
    def attribute(self) -> int:
        ...


def bar(foo: Foo) -> int:
    return foo.attribute()

@attr.define
class CFoo:
    a: int

    def attribute(self) -> int:
        return self.a

bar(CFoo(15))

But if you run it you get:

TypeError: Protocols can only inherit from other protocols, got <class 'attr.AttrsInstance'>

Because in __init__.py it's defined as:

class AttrsInstance:
    pass

instead of

class AttrsInstance(Protocol):
    pass

[euresti]

Ah. Looks like a workaround is to do:

class Foo(Protocol):
    __attrs_attrs__: ClassVar[Any]
   def attribute(self) -> int:
        ...

[hynek]

Is this anything we can fix in attrs?

[euresti]

I think just the small change I said, I can make a PR. But probably not until next week.

It should just be:

from typing import Protocol

class AttrsInstance(Protocol):
    pass