def test_noSystem(self):
        """
        If the new-style C{"log_system"} key is absent, the old-style
        C{"system"} key is set to C{"-"}.
        """
        # Don't use forwardAndVerify(), since that sets log_system.
        event = dict(log_time=time(), log_level=LogLevel.info)
        observed = self.observe(dict(event))
        self.assertEqual(observed["system"], "-")

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