[RET503] Interaction with mypy assert_never

[eddiebergman]

This is rather a niche circumstance but thought it's worth mentioning. For exhaustive checking of Enums, there's an idiom to use with Mypy, checking each possible value and ending with assert_never. This is not a runtime check but purely a Mypy static type check.

Ruff will give a RET503 here which makes sense without mypy but given mypy is telling us it's safe, it would be nice for Ruff to detect this too. In the example above, using ruff with --fix will actually insert the line return None after the assert which then further causes linting issues of unreachable code.

from enum import Enum, auto

from typing_extensions import assert_never


class E(Enum):
    A = auto()
    B = auto()
    C = auto()


def f(value: E) -> bool:
    if value == E.A:
        return True

    if value == E.B:
        return True

    if value == E.C:
        return True

    assert_never(value) # ruff: [RET503] [*] Missing explicit `return` at the end of function able to return non-`None` │ value (ruff) 

My naive solution to this is check for assert_never(.*) where the return should be and accept that as a valid explicit return statement.

Looking into the checker code. It seems like it could be added as a StmntKind to the following lines as I'm guessing this code block is where it says to do nothing.

https://github.com/charliermarsh/ruff/blob/3ed539d50ed6260358c97d2e00b49bad4abfa37e/crates/ruff/src/rules/flake8_return/rules.rs#L272-L275

However I'm really not a rustacian so just grasping at straws.

[NeilGirdhar]

For the purpose of this error, assert False should also do the same thing.

[charliermarsh]

I actually thought we did account for this -- lemme look back at the code today.

[eddiebergman]

Major apologies, latest version does seem to solve this! The releases are just too fast to keep up :)

[charliermarsh]

Oh no worries at all! Glad to hear it :)

[henryiii]

FYI, this is present in cibuildwheel using ruff 0.0.261:

https://github.com/pypa/cibuildwheel/blob/063161f5af6245e8b5c8e3be0ca89fee80ce029e/cibuildwheel/__main__.py#L266

[henryiii]

Is it possible that the code (I think here) isn't accounting for the typing module reexport location?

[charliermarsh]

Ah yeah, you're exactly right.