type hint: how should type of lru_cache be defined?

[david-gang]

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request? bug
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

@lru_cache(maxsize=1)
def get_response_from_api() -> List[ApiObject]:
    url = _get_api_url()
    response = requests.get(url).text
    return json.loads(response, object_hook=_create_api_obj)

• What is the actual behavior/output?

error: Untyped decorator makes function "get_response_from_api" untyped

• What is the behavior/output you expect?
  it should not emit an error, alternatively there should be instructions what to do.

• What are the versions of mypy and Python you are using?
  mypy 0.600
  python 3.6.5
  Do you see the same issue after installing mypy from Git master? did not try

• What are the mypy flags you are using? (For example --strict-optional)
  mypy predictor --ignore-missing-imports --strict

• If mypy crashed with a traceback, please paste
  the full traceback below.

Also asked in SO: https://stackoverflow.com/questions/50495759/type-hint-how-should-type-of-lru-cache-be-defined

[emmatyping]

The underlying issue here is that the type variable for the lru_cache decorator becomes Any since there are no arguments to the function. I think it would be good to handle this case. For now, I'd recommend type ignoring it.

I believe variadic function parameters could help with this.

[gvanrossum]

Well, almost. The type variable only represents the return type; the decorator leaves the arguments unspecified, and the "Untyped decorator makes function "{}" untyped" error (generated by --disallow-untyped-decorators) considers that untyped. Since the function in this case has no arguments it is arguably wrong, but in general the lru_cache decorator does drop the types of the arguments (alas), so `# type: ignore` is still the best you can do ATM.

[ssbarnea]

I don't know if I was just unlucky but I encountered this bug few minutes after attempting to start using mypy for the first time on a project. Lucky me..., I had one function to check, and that one was using @lru_cache.

What is the workaround? (code still supports py27).

[gvanrossum]

Since lru_cache is mostly transparent, maybe this?

from typing import TYPE_CHECKING, TypeVar
if TYPE_CHECKING:
    F = TypeVar('F', Callable)
    def lru_cache(f: F) -> F: pass
else:
    from functools import lru_cache

[OJFord]

Or for the parameterised variant:

    def lru_cache(maxsize: int = 128, typed: bool = False) -> Callable[[F], F]:
        pass

(And note also that F should be bound=Callable.)

[asottile]

no idea how correct this is, but I took a stab at improving typeshed to "fix" this python/typeshed#3880 (didn't work for classes, nevermind)

[EldarSehayekZenity]

What about the ability to access the decorated function in order to access cache info, clear the cache, etc.? The type hints for lru_cache decorator should reflect those are available as well so mypy won't emit errors when accessing them. What do you think?