Type inference issue with "map" and "os.path.normcase"

[deveshks]

The following code raises a type error when run with mypy v0.780.

import os

files = ['foo/BAR', 'FOO/baz']
normcased_files = map(os.path.normcase, files)

The errors are as follows

$ mypy --version
mypy 0.780

$ mypy scratch.py 

scratch.py:4: error: Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[str], AnyStr]"
Found 1 error in 1 file (checked 1 source file)

$ mypy --py2  scratch.py 

scratch.py:4: error: Argument 1 to "map" has incompatible type "Callable[[AnyStr], AnyStr]"; expected "Callable[[str], AnyStr]"
Found 1 error in 1 file (checked 1 source file)

Note that the error is different when run against python 2 vs python 3, even when the type annotations are same for both python 3 and python 2

[Kangaroux]

VScode correctly identifies the sig when you hover: normpath(path: str) -> str

This works:

import os

def fn(s: str) -> str:
    return os.path.normpath(s)

files = ['foo/BAR', 'FOO/baz']
normcased_files = map(fn, files)


reveal_type(os.path.normpath)
# note: Revealed type is 'Overload(def [AnyStr in (builtins.str, builtins.bytes)] (path: builtins._PathLike[AnyStr`-1]) -> AnyStr`-1, def [AnyStr in (builtins.str, builtins.bytes)] (path: AnyStr`-1) -> AnyStr`-1)'

reveal_type(fn)
# note: Revealed type is 'def (x: builtins.str) -> builtins.str'

[Kangaroux]

You can do this as a one line workaround:

normcased_files = map(lambda x: os.path.normpath(x), files)

And if you care about the type:

from typing import List
normcased_files: List[str] = list(map(lambda x: os.path.normpath(x), files))

[Kangaroux]

With regards to the comment on #8962, since the map function is using overloads, did you try mypy 0.770 to see if that works?

[deveshks]

You can do this as a one line workaround:

Thanks. I was aware of the workaround, but that's not the actual problem here. The original code should also pass type checks.

With regards to the comment on #8962, since the map function is using overloads, did you try mypy 0.770 to see if that works?

No, that doesn't work either

$ mypy --version
mypy 0.770

$ mypy scratch.py 
scratch.py:4: error: Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[str], AnyStr]"

[AlexWaygood]

Closing as a duplicate of #2389 (although the example is slightly different, the bug appears to be to be the same)