[mehdimirzafaruk]

When a mock created by mocker.patch (or patch.object) has __enter__, pytest-mock sets a side effect on it that warns about using the mock as a context manager. That side effect is a lambda returning the result of warnings.warn(), which is None, and since a side effect that returns something other than DEFAULT replaces the return value, entering the patched object always gives None:

class dummy_module:
    pool = None

def test_pool(mocker):
    pool = mocker.patch.object(dummy_module, "pool")
    pool.__enter__.return_value = connection = object()
    with dummy_module.pool as entered:   # PytestMockWarning, which the message says can be ignored
        pass
    assert entered is connection         # fails: entered is None

With unittest.mock.patch.object the same code gives connection. This shows up when the patched object is itself the context manager (a lock, a pool, a session), where the warning message says it "can be ignored", but the configured __enter__.return_value is silently lost too.

The fix makes the warning side effect return DEFAULT, so __enter__ still warns and then returns its return_value as usual.

Tests: added TestContextManagerWarnings::test_patching_a_context_manager_object, which fails on main (assert None is <MagicMock name='pool.__enter__()'>) and passes with the fix. tox -e py314,norewrite passes and pre-commit is clean.

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