correct parse_args namespace attribute

[PrajwalM2212]

fixes #2366

This PR allows parse_args namespace arg to accept any object and also to return any object

[srittau]

This is not the right solution. There are basically three cases that can be distinguished using @overload:

1. No namespace argument given -> return Namespace
2. namespace is None -> return Namespace
3. namespace is any other value: return the same type. This can be achieved using a TypeVar.

[srittau]

This looks much better. Two issues, though.

[srittau]

You need to remove the default from the namespace parameter here. Otherwise type checkers get confused when calling parse_args([]), because both overloads match.

Also, the namespace passed in is returned unchanged, not used as a constructor. So the annotation for namespace should be just _N, not Type[_N].

[srittau]

Addendum: If you remove the default here, you should not need the third overload you just added.

[PrajwalM2212]

When I remove the default by setting namespace: _N , pycharm high-lightens it as non-default parameter follows default parameter. Should I ignore it ?

[srittau]

Ah, my bad. Try these two lines instead of the one above:

@overload
def parse_args(self, args: Optional[Sequence[_Text]], namespace: Type[_N]) -> _N: ...
@overload
def parse_args(self, *, namespace: Type[_N]) -> _N: ...

[JelleZijlstra]

I think this overload shouldn't have the namespace argument. If namespace is given, the second overload suffices.

[PrajwalM2212]

Closing ! Will reopen this when I understand more about how overload works in mypy

[PrajwalM2212]

@srittau @JelleZijlstra Thanks for your generous help. I will reopen this when I understand more about how overload works in mypy.

[srittau]

@PrajwalM2212 Thank you for your effort to fix those small bugs, I really appreciate it! I think you were nearly there with this PR. Could you try this:

    _N = TypeVar('_N')
    @overload
    def parse_args(self, args: Optional[Sequence[_Text]] = ...) -> Namespace: ...
    @overload
    def parse_args(self, args: Optional[Sequence[_Text]], namespace: _N) -> _N: ...
    @overload
    def parse_args(self, *, namespace: _N) -> _N: ...

This changes two things compared to your last version:

• It moved the typevar before the first overload. It seems type checkers don't like it if it's between the overloads. You could also move the typevar to the top of the file, which we usually do
• Remove Type around the namespace argument (see my previous comment).

[srittau]

I am sorry that it failed again. It seems that pytype does not like typevars inside classes. I didn't know that. So just move it to the start of the file and then it should finally pass! Thank you for your persistence.