def test_GID(self):
        """
        Check UID/GID switches when current effective GID is non-root.
        """
        self._testUIDGIDSwitch(0, 1, 0, 0, [], [0, 1])
        self._testUIDGIDSwitch(0, 1, 0, 1, [], [])
        self._testUIDGIDSwitch(0, 1, 1, 1, [1, 0], [])
        self._testUIDGIDSwitch(0, 1, 1, 2, [1, 0], [2, 1])

        Check UID/GID switches when current effective GID is non-root.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/test/test_util.py