def test_trace(self):
        """
        Tracing keeps track of forwarding to the publisher.
        """
        def publisher(event):
            observer(event)

        def observer(event):
            self.assertEqual(event["log_trace"], [(log, publisher)])

        log = TestLogger(observer=publisher)
        log.info("Hello.", log_trace=[])

        Tracing keeps track of forwarding to the publisher.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_logger.py