functools.lru_cache doesn't work with generic functions

[rwbarton]

from typing import TypeVar
import functools

T = TypeVar('T')

@functools.lru_cache()
def f(x: T) -> T: pass

reveal_type(f) # E: Revealed type is 'functools._lru_cache_wrapper[T`-1]'
reveal_type(f(1)) # E: Revealed type is 'T`-1'

These types are wrong: types are not allowed to contain unbound type variables.

There isn't really a clearly correct thing to do here though. The stubs for lru_cache are defined in a way that does not retain the relationship between the input and output types of the function being decorated. Options here include

• Find a way to retain the entire Callable type of the decorated function while having the result of lru_cache's __call__ simultaneously be an instance of _lru_cache_wrapper. As far as I can tell, this is currently impossible, but would be the ideal long-term outcome.

• Give up on _lru_cache_wrapper and CacheInfo (for now?) and just give lru_cache the type

  def lru_cache(f: _TC, maxsize: int = ..., typed: bool = ...) -> _TC: ...

  where _TC = TypeVar(_TC, bound=Callable), basically reverting python/typeshed@a40418e.

• Have mypy infer the type _lru_cache_wrapper[Any] for this f on the grounds that the type variable _T is undetermined. But normally in this situation (where an undetermined type variable appears in an inferred type) mypy either uses 'None' or complains about being unable to infer a type. What makes this case special?

This kind of situation occurs in stdlib-samples/3.2/fnmatch.py:

@functools.lru_cache(maxsize=250)
def _compile_pattern(pat: AnyStr,
                     is_bytes: bool = False) -> Callable[[AnyStr],
                                                         Match[AnyStr]]:
    ...

It accidentally happens to not cause an error at the uses of _compile_pattern because of another issue, #1261.

[JukkaL]

Interestingly, this is an example where intersection types would be useful. We could annotate lru_cache like this:

def lru_cache(
    f: _TC, 
    maxsize: int = ..., 
    typed: bool = ...) -> Intersection[_TC, <wrapper type>]: ...

[ilevkivskyi]

I think this is a duplicate of #1317