list doesn't seem to be a valid callable

[Akuli]

don't know if this is bug or feature request

def foo() -> None:
    iterator = map(list, ['hello', 'world'])  # error: Argument 1 to "map" has incompatible type "Type[List[Any]]"; expected "Callable[[str], List[_T]]"
    reveal_type(iterator)

this works, but feels like a workaround:

iterator = map(lambda foo: list(foo), ["hello", "world"])

python 3.7, mypy from latest master (20b4628), no flags

[gvanrossum]

I’m sure this is well known and probably a duplicate. You can help by searching the tracker for an issue that describes this.

[Akuli]

couldn't find another issue quite like this, but i tried other callable builtin classes

map(dict, ['12', '34'])       # error
map(enumerate, ['12', '34'])  # error
map(float, ['12', '34'])      # ok
map(int, ['12', '34'])        # ok
map(list, ['12', '34'])       # error
map(str, ['12', '34'])        # ok
map(tuple, ['12', '34'])      # error

seems to be problem with generics? no generic classes work and all non-generics work

[gvanrossum]

Hm, that's interesting. Seems only the generic classes have this error. Maybe that's a clue for someone.

[Akuli]

It could be the "known very old bug that generic functions when passed as arguments to another generic functions don't work". It seems that nobody can find that from the issue tracker, but some people know about it. #9265 (comment)

[JukkaL]

#1507 is a similar issue, though maybe we can keep this open as well, since the fix for type objects might be different and the error message looks different.

[tiagocabo]

I have a similar issue when I try
map(str, [1, 2])

having this error
Incompatible types in assignment (expression has type "map[str]", variable has type "Optional[List[int]]")

[Akuli]

@tiagocabo That is not a bug: map doesn't return a list, it returns an iterator, and you are trying to assign it to a list variable. You can annotate the variable you're assining map(str, [1, 2]) to as

variable_name: Optional[Iterable[int]]

or (with Python 3.10+ or from __future__ import annotations)

variable_name: Iterable[int] | None

Alternatively, you can convert the iterator to a list with list(map(str, [1, 2])).