lru_cache-wrapped function args must be Hashable

[ikonst]

Per docs:
Since a dictionary is used to cache results, the positional and keyword arguments to the function must be hashable.

[hauntsaninja]

A little wary of this change; type checkers don't really understand Hashable: #3884 (comment)

[ikonst]

Thanks, I learned something :) I've indeed wondered how Hashable worked in mypy since object is hashable.

I tried this approach of restricting arg types to Hashable, and mypy did catch (thanks to #3219) passing list, dict etc. which would otherwise raise in runtime. So it does seem to add some value without adding false negatives. no?

BTW, Another thing that irks me about this definition is that it doesn't maintain the wrapped function's signature, i.e. it's not

_T = TypeVar('_T', bound=Callable)

def lru_cache() -> Callable[[_T], _T]: ...

Of course defining it this way would do away with the wrapper's methods like cache_clear method, which I wouldn't want to do.

[JelleZijlstra]

I might have been too pessimistic about Hashable in the past; mypy does understand it now. This PR looks like a step in the right direction.