def test_failure(self):
        """
        Captured failures in the new style set the old-style C{"failure"},
        C{"isError"}, and C{"why"} keys.
        """
        event = self.eventWithFailure()
        self.assertIs(event["failure"], event["log_failure"])
        self.assertTrue(event["isError"])
        self.assertEqual(event["why"], "oopsie...")

        Captured failures in the new style set the old-style C{"failure"},
        C{"isError"}, and C{"why"} keys.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_legacy.py