lru_cache is not compatible with mypy when decorated method

[dimastbk]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.12.0

Python version

3.13.8

What happened?

Hi. When decorating a method on a subclass, mypy raises an error. async_lru is fine with it.

How can we reproduce the bug?

import abc

from anyio.functools import lru_cache
from async_lru import alru_cache


class Iterface(abc.ABC):
    @abc.abstractmethod
    async def method(self, value: str) -> str: ...


class AnyIO(Iterface):
    @lru_cache()
    async def method(self, value: str) -> str:  # error
        return value


class AsyncLru(Iterface):
    @alru_cache()
    async def method(self, value: str) -> str:
        return value


async def main() -> None:
    anyio_value = await AnyIO().method("test")  # error
    async_lru_value = await AsyncLru().method("test")

    assert anyio_value == async_lru_value

test.py:14: error: Signature of "method" incompatible with supertype "Iterface"  [override]
test.py:14: note:      Superclass:
test.py:14: note:          def method(self, value: str) -> Coroutine[Any, Any, str]
test.py:14: note:      Subclass:
test.py:14: note:          AsyncLRUCacheWrapper[[AnyIO, str], Any]
test.py:25: error: Missing positional argument "value" in call to "__call__" of "AsyncLRUCacheWrapper"  [call-arg]
test.py:25: error: Argument 1 to "__call__" of "AsyncLRUCacheWrapper" has incompatible type "str"; expected "AnyIO"  [arg-type]

mypy 1.18.2 (compiled: yes)

[agronholm]

It looks like a problem with how it attaches to instance methods.

[agronholm]

Also AnyIO is using more precise annotations (ParamSpec) than async_lru, that could contribute to the issue.

[tapetersen]

This is not only a typing issue but how it works, or rather doesn't work, with methods in general.

import anyio
from anyio import functools

class A:
    @functools.lru_cache
    async def meth(self, a: int) -> int:
        return a

async def main():
    a = A()
    assert await a.meth(1) == 1

anyio.run(main)

The wrapper needs to implement the Descriptor behavior of binding self a normal function has for that to work. (Essentially implementing __get__ returning a new wrapper with the first argument bound.

I've had to do something similar and it's doable but getting it to work seamlessly with typing isn't trivial.

As is It should probably be documented to not work on methods, at least not as people expect, or right out just fail.

[agronholm]

I've been working on a fix and I implemented the descriptor stuff but the typing part is giving me grief.

[tapetersen]

Looking at typeshed they just give up and use *args **kwargs https://github.com/python/typeshed/blob/8d96801533918957fb194e101cb321bfe1f836f8/stdlib/functools.pyi#L57

[tapetersen]

I could take a jab at the typing if you can share the partial work of implementation.

[agronholm]

I could take a jab at the typing if you can share the partial work of implementation.

I pushed the fix-1042 branch where I made it work at run-time but typings are still a mess.