Default non-optional function parameter is accepted with a default as None.

[Jiehong]

Hi,

Mypy seems to not be bothered by None as a default function argument, even if that argument is not an Optional.

Here is an example:

def type_check_me(index: int = None) -> str:
    return f"{index}"

Running mypy on this does not lead to any issue.

mypy version: 0.782

Expected:

I would expect mypy to tell me that None is not a valid default value for index, because it has type int.

Instead, the following code should be considered valid:

from typing import Optional

def type_check_me(index: Optional[int] = None) -> str:
    return f"{index}"

[Jiehong]

Note: I tried with --strict-optional (even though it's the default now), with no difference.

[kaste]

I think you're looking for --no-implicit-optional https://mypy.readthedocs.io/en/stable/config_file.html#none-and-optional-handling

[JelleZijlstra]

Maybe we should flip the default on --no-implicit-optional?

[JukkaL]

Maybe we should flip the default on --no-implicit-optional?

Yeah, I've been thinking about this. We might want to do it separately from other major breaking changes, such as the switch to a modular typeshed.

[JukkaL]

Closing in favor of #9091. Let's continue the discussion there.