Is there correct way to use python typing?

Question

I'm trying to figure out typing in Python and I'm having trouble with the following code:

from collections.abc import Iterable
from typing import TypeVar

T = TypeVar('T')

def convert_to_iter(var: T | Iterable[T]) -> Iterable[T]:
    if not isinstance(var, Iterable):
        return (var, )
    return var

I am using Pylance in VScode with typeCheckingMode: "strict" and am getting the error on the last line of the code:

Return type, "Iterable[Unknown]* | Iterable[T@convert_to_list]", is partially unknown Pylance(reportUnknownVariableType)

Can someone please explain why this is incorrect?

Answer 1

There is a discussion in this issue about such question. So, this behavior can be explained by type narrowing. One of the existing ways to eliminate this error is to use cast function