Unresolved generic types

[AdrianSosic]

Hi @Tinche, could it be that there is a line for type resolution missing somewhere in cattrs? I just noticed the situation shown below, which doesn't occur if I add an explicit resolve_types call (see commented code lines). If this is expected behavior, I'd appreciate if you could briefly comment on what's the issue. Thanks 🙃

from __future__ import annotations

from typing import Any, Generic, Protocol, TypeVar

import cattrs
from attrs import define, field

T = TypeVar("T")


class SomeProtocol(Protocol):
    pass


@define
class ImplementsProtocol(SomeProtocol, Generic[T]):
    template: T = field()


# --> Uncomment this and the examples works
# from attrs import resolve_types
# resolve_types(ImplementsProtocol)


@define
class ContainsProtocol:
    container: SomeProtocol = field()


converter = cattrs.Converter()


@converter.register_unstructure_hook
def _unstructure_container(obj: SomeProtocol):
    return converter.unstructure(obj, unstructure_as=ImplementsProtocol[int])


c = ContainsProtocol(ImplementsProtocol(42))
print(converter.unstructure(c))

TypeError: __main__.ImplementsProtocol[int] is not a module, class, method, or function.

[Tinche]

Here's a minimized example:

from __future__ import annotations

from typing import Generic, TypeVar

import cattrs
from attrs import define

T = TypeVar("T")


@define
class ImplementsProtocol(Generic[T]):
    template: T


converter = cattrs.Converter()


print(
    converter.unstructure(
        ImplementsProtocol(42), unstructure_as=ImplementsProtocol[int]
    )
)

Looks like a bug, most likely here: https://github.com/python-attrs/cattrs/blob/main/src/cattrs/converters.py#L1249 (line should be: resolve_types(origin or cl). I can get it in a day or two, or if you want to (the trickiest part will be adding a test :p)

[AdrianSosic]

Glad to hear it's a bug and not my wrong understanding 🐈 Re test: what do you have in mind? Why not just use your minimized example as test?

[Tinche]

Ah actually you're right. The class should just be defined in a file that has from __future__ import annotations. This used to be more annoying when we still supported Python versions without this feature :p