Add a `set_filter()` context manager to the caplog fixture

[jenstroeger]

What's the problem this feature will solve?

I need to test a custom filter for logging, and currently I use one of the following two approaches:

def test_filter(caplog: pytest.LogCaptureFixture) -> None:
    caplog.set_level(logging.INFO)

    logger = logging.getLogger(__name__)  # This test module's logger.
    filter_ = SchnufteCustomFilter()
    logger.addFilter(filter_)

    # Run tests.
    assert len(caplog.record_tuples) == 1
    assert caplog.record_tuples[0] == ("test_custom_filter", 20, "Schnufte!")

    logger.removeFilter(filter_)

or

def test_filter(caplog: pytest.LogCaptureFixture) -> None:
    caplog.set_level(logging.INFO)

    filter_ = SchnufteCustomFilter()
    caplog.handler.addFilter(filter_)

    # Run tests.
    assert len(caplog.record_tuples) == 1
    assert caplog.record_tuples[0] == ("test_custom_filter", 20, "Schnufte!")     

    caplog.handler.removeFilter(filter_)

Describe the solution you'd like

Similar to the at_level() context manager I think it would be useful to wrap the second of the above filter setups into a custom context manager, that I can then call like so:

def test_filter(caplog: pytest.LogCaptureFixture) -> None:
    caplog.set_level(logging.INFO)

    filter_ = SchnufteCustomFilter()
    with caplog.set_filter(filter_):  # use_filter() or with_filter() or ...
        # Run tests.
        assert len(caplog.record_tuples) == 1
        assert caplog.record_tuples[0] == ("test_custom_filter", 20, "Schnufte!")     

Alternative Solutions

See above’s examples, I guess.

Additional context

I’m happy to give a PR a shot: I suppose the change would be adding a method to the LogCaptureFixture, for example:

@final
class LogCaptureFixture:

    @contextmanager
    def set_filter(filter_: logging.Filter) -> Generator[None, None, None]:  # Type needs to be aligned with _FilterType from stdlib/typeshed.
        self.handler.addFilter(filter_)
        try:
            yield
        finally:
            self.handler.removeFilter(filter_)

[nicoddemus]

👍 from me.

Bikesheding:

with caplog.with_filter(SchnufteCustomFilter()): ...

with caplog.filtering(SchnufteCustomFilter()): ...

I prefer the latter, reads naturally to me (although I'm not a native speaker).

[jenstroeger]

@nicoddemus shall I go ahead and create a PR?

[nicoddemus]

Yes please

[jenstroeger]

@nicoddemus what’s the recommendation regarding testing: the at_level() context manager has its own test functions here, so I could add the new filtering() context manager into those existing functions, or create new functions?

[nicoddemus]

New functions are preferred. 👍