argparse.ArgumentParser.add_argument doesn't allow type=os.path.abspath

[andymwood]

Running mypy on this code:

import argparse,  os
reveal_type(os.path.abspath)
p = argparse.ArgumentParser()
p.add_argument("file", type=os.path.abspath)

I get:

mypy args.py 
args.py:2: 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)'
args.py:4: error: Argument "type" to "add_argument" of "_ActionsContainer" has incompatible type overloaded function; expected "Union[Callable[[str], AnyStr], FileType]"

Should the type signature of add_argument be changed to allow functions that accept a PathLike argument?

[srittau]

I assume you are using Python 3.6+. If I'm not mistaken, the stubs are correct. Callable[[str], _T] matches the overload def basename(path: AnyStr) -> AnyStr: ... with AnyStr and _T constrained to str. Possibly a not supported edge case in mypy? pytype accepts this.

[andymwood]

Ok, thanks for that. I've reported the issue at the python/mypy repo instead.

[andymwood]

I'm re-opening this issue following the advice of this comment.

[srittau]

I still think add_argument is typed correctly. Replacing _T with Any would actually reduce the strictness, since _T is deliberately used for both type and choices. By changing _T to Any, this wouldn't be caught:

from argparse import ArgumentParser

def to_str(x: str) -> str:
    return x

arg = ArgumentParser()
arg.add_argument(type=to_str, choices=[1])

CC @ilevkivskyi

[ilevkivskyi]

The fact that mypy currently flags above code is a bug in mypy (quite subtle one). It should infer _T = object for the method and accept this call.

For example this code is correctly accepted:

T = TypeVar('T')
def g(x: T, y: T) -> None: ...
g('yes', 42)

[JelleZijlstra]

Pyright allows the code in the original post and it also looks fine to me with the current stub, so I am going to say this is still a bug in mypy.