def test_formatEmptyEventWithTraceback(self):
        """
        An event with an empty C{log_format} key appends a traceback from
        the accompanying failure.
        """
        try:
            raise CapturedError("This is a fake error")
        except CapturedError:
            f = Failure()
        event = {
            "log_format": u""
        }
        event["log_failure"] = f
        eventText = eventAsText(
            event,
            includeTimestamp=True,
            includeSystem=False
        )
        self.assertIn(unicode(f.getTraceback()), eventText)
        self.assertIn(u'This is a fake error', eventText)

        An event with an empty C{log_format} key appends a traceback from
        the accompanying failure.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_format.py