map(os.path.basename, ...): Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[str], AnyStr]"

[jdufresne]

$ python --version
Python 3.9.1
$ mypy --version
mypy 0.790

Minimal code example:

import os

filenames = map(os.path.basename, ["hello/world.txt"])

Actual result:

$ mypy ~/test.py
/home/jon/test.py:3: error: Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[str], AnyStr]"
Found 1 error in 1 file (checked 1 source file)

Expected: No error. This seems like an acceptable use of os.path.basename.

---

I'm happy to lend a hand to resolve this issue, but it is not clear to me how to fix it. Is this an issue with os.path.basename or map or something else?

[srittau]

This is a reduced version of the example above that shows the problem without involving typeshed types (or overloads):

from typing import Callable, List, TypeVar

_B = TypeVar("_B")
_T = TypeVar("_T")
_S = TypeVar("_S")

def bar(x: _B) -> _B:
    return x

def foo(func: Callable[[_T], _S], y: _T) -> _S:
    return func(y)

foo(bar, "")  # <-- error

This results in:

foo.py:13: error: Argument 1 to "foo" has incompatible type "Callable[[_B], _B]"; expected "Callable[[str], _B]"
Found 1 error in 1 file (checked 1 source file)

I believe this is a mypy problem. Could a mypy maintainer move this issue?

In typeshed we could probably work around the problem by adding additional overloads for str and bytes to basename(), but I would obviously prefer this to be solved in mypy.

[srittau]

Possibly related: #3644.

[hauntsaninja]

Also possibly related: #5738 (comment)

[AlexWaygood]

Similar to #2389, but I'm not sure that it's a dupe.