operator.itemgetter can accept any hashable type

[berdario]

python/mypy#2153

basically, the following is valid Python code:

>>> itemgetter(Ellipsis)({Ellipsis: -1})
-1

in my case I got the error message with a more mundane string key, thus yielding this error:

 No overload variant of "itemgetter" matches argument types [builtins.str]

[gvanrossum]

Oh, there are lots of interesting things we could do here. Maybe it should be a generic function, e.g.

@overload
def itemgetter(item: int) -> Callable[[Sequence[T]], T]: ...
@overload
def itemgetter(item: K) -> Callable[[Mapping[K, V], V]: ...

except the latter doesn't work yet because it introduces a type variable in the (callable) return type (I think the reason is related to python/mypy#1927). The *items case is even more interesting, see python/typing#239.

So maybe for now we need something with a lots of Any (or for the arguments object) and a TODO comment.

[berdario]

Even if it would work, doesn't overload resolution rely only on the input arguments?

In that case

itemgetter(1)({1: 42})

would fail to typecheck, due to the the returned function expecting a Sequence, not a Mapping (unless Sequence[X] are Mapping[int, X] ?)

[gvanrossum]

Yeah, that's one of the challenges. It needs something that supports
getitem but there's no ABC for that...

--Guido (mobile)

[gvanrossum]

Fixed by #564. (Note: if you include "Fixes #558" in the body of your commit message it will auto-close the issue.)