"Never" is not iterable

[GammaGames]

I'm writing a fixture for pytest that returns an object with an optional kwarg. It's reporting a problem even though I've checked if the variable isn't None and made sure it's a list.

Environment data

• Pylance version: v2024.9.1
• OS and version: Ubuntu 22.04 LTS
• Python version (& distribution if applicable, e.g. Anaconda): 3.10.12

Code Snippet

def fixture():
    _arg = None
    class FixtureClass:
        def do_thing(self, arg=None):
            nonlocal _arg
            if arg is not None:
                _arg = arg

    _class = FixtureClass()
    if _arg is not None:
        if isinstance(_arg, list):
            for s in _arg:
                _class.do_thing(s)
        else:
            _class.do_thing(_arg)

Repro Steps

1. Create a new python file and paste the code in

Expected behavior

No problems

Actual behavior

[debonte]

Pylance is behaving properly here. _arg is always None within fixture(). Therefore, _arg is not None will always be false. That's why _arg is narrowed to Never within that if block -- there's no type that _arg could possibly have in that context.

[GammaGames]

Makes sense, thank you! I should declare it with _arg : str | list | None = None instead