E1129 false positive with match

[dfioravanti]

Bug description

If one uses a match statement inside a context manager pylint complains that there is a 'NoneType' context manager. The following is the smallest code I managed to cook up that reproduce the problem

from contextlib import contextmanager
from enum import Enum, auto
from typing import ContextManager, Any


class Cases(Enum):
    CASE1 = auto()
    CASE2 = auto()


@contextmanager
def context_manager_with_if(whatever: Any, case: Cases) -> ContextManager[Any]:
    if case == Cases.CASE1:
        print(1)
        yield whatever
    elif case == Cases.CASE2:
        print(2)
        yield whatever
    else:
        raise ValueError("Wrong case")


@contextmanager
def context_manager_with_match(whatever: Any, case: Cases) -> ContextManager[Any]:
    match case:
        case Cases.CASE1:
            print(1)
            yield whatever
        case Cases.CASE2:
            print(2)
            yield whatever
        case _:
            raise ValueError("Wrong case")


if __name__ == "__main__":

    WHATEVER = 23
    with context_manager_with_if(WHATEVER, case=Cases.CASE1) as context:
        print(context)
    with context_manager_with_match(WHATEVER, case=Cases.CASE1) as context:
        print(context)

Configuration

No response

Command used

pylint tmp/example.py

Pylint output

************* Module example
tmp/example.py:41:4: E1129: Context manager 'NoneType' doesn't implement __enter__ and __exit__. (not-context-manager)

Expected behavior

To me that looks like a valid context manager. It should not raise anything

Pylint version

pylint 2.16.1
astroid 2.14.2
Python 3.11.1 (main, Dec 23 2022, 09:28:24) [Clang 14.0.0 (clang-1400.0.29.202)]

OS / Environment

MacOS 13.2 and Debian 11, it happens both inside and outside docker

Additional dependencies

No response

[jacobtylerwalls]

Thanks for the report. pylint's inference engine astroid doesn't understand the returns from functions using match case. See:

>>> import astroid
>>> code = """
... def a(case):
...   match case:
...     case 1:
...       return 42
... 
... aa = a(1) #@
... """
>>> assign_node = astroid.extract_node(code)
>>> assign_name_node = assign_node.targets[0]
>>> assign_name_node.inferred()
[<Const.NoneType l.None at 0x10d477c90>]

Closing as duplicate of #5288 (and re-titling it)

[dfioravanti]

Ah interesting, good to know, thanks for the explanation