def f() -> AnyStr fails if f returns a str

[andymwood]

I expect mypy to allow the following code to pass:

from typing import AnyStr

def f() -> AnyStr:
    return "abc"

Instead, I get this:

mypy anystr.py
anystr.py:5: error: Incompatible return value type (got "str", expected "bytes")

I'm using Python 3.6 and mypy 0.711

[JelleZijlstra]

Did you mean to use def f() -> Union[str, bytes]:? What you write doesn't really make sense because there's nothing to constrain the value of the TypeVar.

[andymwood]

Thanks for that. It seems I'd misunderstood something. However, there's another possible bug which might have misled me into using AnyStr.

The following is closer to what I was originally trying to do:

import os
from typing import Dict, Iterable


def get_files() -> Iterable[os.DirEntry]:
    for f in os.scandir():
        yield f


def get_dict() -> Dict[str, str]:
    return {"a": "b"}


files = get_files()
x = get_dict()
for key in map(os.path.realpath, files):
    val = x.get(key)
    print(key, val)

mypy returns this error:

anystr.py:17: error: Argument 1 to "get" of "Mapping" has incompatible type "AnyStr"; expected "str"

The error goes away if I change map(os.realpath, files) to a list or generator comprehension, or call os.path.realpath inside the loop like so:

for f in files:
    key = os.path.realpath(f)

[JelleZijlstra]

I think this is because you're missing a type argument on os.DirEntry; if you can mypy with --disallow-any-generics (a very useful option), mypy would likely tell you that. Try making get_files() return "Iterator[os.DirEntry[str]]" (you need the quotes because DirEntry is not generic at runtime).

[andymwood]

Following your advice, I get this:

mypy --disallow-any-generics anystr.py
anystr.py:16: error: Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[DirEntry[str]], AnyStr]"
anystr.py:17: error: Argument 1 to "get" of "Mapping" has incompatible type "AnyStr"; expected "str"

[ilevkivskyi]

The original problem is not a bug, as Jelle explained. The second issue is a duplicate of #1317, passing one generic function (os.path.realpath in this case) as an argument to another generic function (map in this case) doesn't really work unfortunately.