mypy and pylint disagree about uselessness of return statements

[clo-vis]

Bug description

from contextlib import AbstractContextManager
from types import TracebackType
from typing import Optional, Type


class Playground(AbstractContextManager["Playground"]):
    def __exit__(
        self,
        exc_type: Optional[Type[BaseException]],
        exc_value: Optional[BaseException],
        traceback: Optional[TracebackType],
    ) -> Optional[bool]:
        print("closing what needs to be closed")
        return None

Configuration

No response

Command used

pylint playground.py

Pylint output

src\playground.py:7:4: R1711: Useless return at end of function or method (useless-return)

Expected behavior

No error message, because without return None mypy complains:

error: Missing return statement [return]

And mypy swears that they are right, see for example python/mypy#3974

(By the way, the signature from exit was copied from https://mypy.readthedocs.io/en/stable/protocols.html#contextmanager-t)

Pylint version

pylint 2.13.8
astroid 2.11.4
Python 3.9.5 (tags/v3.9.5:0a7dcbd, May  3 2021, 17:27:52) [MSC v.1928 64 bit (AMD64)]

OS / Environment

No response

Additional dependencies

No response

[Pierre-Sassoulas]

I think both tools are right according to pep8 : Either all return statements in a function should return an expression, or none of them should.

If you don't want to return anything use -> None instead of -> Optional[bool], mypy will be happy :)

[mbyrnepr2]

To add to Pierre's comment, there is also an instructive docstring relating to this:
https://github.com/PyCQA/pylint/blob/main/pylint/checkers/refactoring/refactoring_checker.py#L1866

[Pierre-Sassoulas]

Closing as answered.

[clo-vis]

If you don't want to return anything use -> None instead of -> Optional[bool], mypy will be happy :)

A better answer is "use # type: ignore and mypy will be happy.

Even though for the example above using -> None does the job, you cannot always just replace the type of the return value with -> None and expect that mypy won't complain, as in

from typing import overload, Union, Literal


@overload
def fetch_data(parse: Literal[True]) -> int:
    ...


@overload
def fetch_data(parse: Literal[False]) -> None:
    ...


@overload
def fetch_data(parse: bool) -> Union[int, None]:
    ...


def fetch_data(parse: bool) -> Union[int, None]:  # type: ignore[return]
    print(parse)