argparse.ArgumentParser.parse_args() overload misfire

[gvanrossum]

I had some code calling parse_args(args, ns) where the type of ns was Optional[Namespace. It then deduced that the return type was also Optional[Namespace]. I think this is a mistake -- I don't think parse_args() ever returns None. Perhaps _N should be constrained by bound='Namespace'?

[srittau]

The argparse documentation gives an explicit example of a non-Namespace namespace argument. Personally, I prefer a false negative over a false positive, especially in this case.

[gvanrossum]

OK, but can we at least do something so that mypy understands that parse_args(args, None) does not return None? Surely that's a special case worth handling. My example code is roughly

def parse_args_wrapper(argv: List[Text], namespace: Optional[Namespace]) -> Namespace:
    return ...parse_args(argv, namespace)

and this gives an error on the return saying that it expects Namespace but got an Optional[Namespace] -- I can only explain this by asuming that this is because _N is unconstrained in the second and third overloads.

Maybe adding another overload with an explicit None will help?

[srittau]

Explicit None overloads are definitely missing, but would not help your use case, as far as I can tell, since that case would not match. Best idea I have is an overload for Optional[_N] -> Any, where Any is really Union[Namespace, _N]. (Another case the AnyOf (python/typing#566) would be useful.)

[gvanrossum]

Hm, I'm not so sure. Here's a simple test program:

from argparse import ArgumentParser, Namespace
from typing import Optional

def foo(p: ArgumentParser, ns: Optional[Namespace]) -> Namespace:
    return p.parse_args(namespace=ns)

This currently gives

t.py:5: error: Incompatible return value type (got "Optional[Namespace]", expected "Namespace")

But when I insert

    @overload
    def parse_args(self, *, namespace: None) -> Namespace: ...

in the list of parse_args() overloads (before the last one), it passes without errors.

(We'd also need an extra overload for (Optional[Sequence[_Text]], None) -> Namespace.)

[srittau]

Huh, learned something new today. That's definitely useful and in that case just overloading None seems like the correct fix.

[ilevkivskyi]

Union math kicks in soon on purpose, mypy now works much better with overloads :-)