def test_system(self):
        """
        The old-style C{"system"} key is copied to the new-style
        C{"log_system"} key.
        """
        publishToNewObserver(
            self.observer, self.legacyEvent(), legacyLog.textFromEventDict
        )
        self.assertEqual(
            self.events[0]["log_system"], self.events[0]["system"]
        )

        The old-style C{"system"} key is copied to the new-style
        C{"log_system"} key.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_legacy.py