[1.7] Any in decorated function with TypeVar

[Dreamsorcerer]

Bug Report

This looks different to me, but could maybe be the same as #16481.

aiohttp_admin/security.py:18:1: error: Type of decorated function contains type
"Any" ("_lru_cache_wrapper[TypeAdapter[Any]]")  [misc]
    def _get_schema(t: Type[_T]) -> TypeAdapter[_T]:
    ^
aiohttp_admin/security.py:24:5: error: Returning Any from function declared to
return "_T"  [no-any-return]
        return _get_schema(t).validate_python(value)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Found 2 errors in 1 file (checked 20 source files)

https://github.com/aio-libs/aiohttp-admin/actions/runs/6845242250/job/18610174092?pr=810

To Reproduce

from collections.abc import Hashable
from functools import lru_cache
from typing import Type, TypeVar

from pydantic import TypeAdapter

_T = TypeVar("_T", bound=Hashable)


@lru_cache
def _get_schema(t: Type[_T]) -> TypeAdapter[_T]:
    return TypeAdapter(t)


def check(t: Type[_T], value: object) -> _T:
    """Validate value is of static type t."""
    return _get_schema(t).validate_python(value)

Also at aio-libs/aiohttp-admin#810

Or, without pydantic:

from collections.abc import Hashable
from functools import lru_cache
from typing import Type, TypeVar

_T = TypeVar("_T", bound=Hashable)


@lru_cache
def _get_schema(t: Type[_T]) -> list[_T]:
    return []


def check(t: Type[_T], value: object) -> _T:
    """Validate value is of static type t."""
    return _get_schema(t)[0]

Expected Behavior

There shouldn't be Any in these functions.

[ilevkivskyi]

lru_cache has a bunch of Anys. You can add some reveal_type()s to see what is going on.

[Dreamsorcerer]

@ilevkivskyi I think you're mistaken. It's replacing my TypeVar with Any, and throwing away the type information for my function's return type. This is a regression in 1.7.

Look specifically at the error output, the first error says TypeAdapter[Any]. The second shows that any time I use _get_schema() it returns Any.

I just realised I forgot to include the arguments, but this reproduces with mypy --strict --disallow-any-decorated.

[ilevkivskyi]

I understand this is counterintuitive, but taking into account signature of lru_cache this is correct behavior. For example, the inferred type of _get_schema is _lru_cache_wrapper[list[Any]], previously mypy inferred some nonsense type like _lru_cache_wrapper[list[_T]], and this is why this error was absent.

[Dreamsorcerer]

Yep, OK, I just went through typeshed and realised it made sense, but not why it appeared to work in older versions. Seems like it should use ParamSpec or something...

[Dreamsorcerer]

Ah, that was already tried. If anybody else wants to track the current state of lru_cache, it's in python/typeshed#6347

[AlexWaygood]

Ah, that was already tried. If anybody else wants to track the current state of lru_cache, it's in python/typeshed#6347

Yeah, it's unfortunately a pretty difficult problem to fix in typeshed. There have been many attempts.

[Dreamsorcerer]

I would note that the discussion around this issue appears to be waiting for some input from the mypy side though:
python/typing#946 (reply in thread)