def test_formatEventUnformattableTraceback(self):
        """
        If a traceback cannot be appended, a message indicating this is true
        is appended.
        """
        event = {
            "log_format": u""
        }
        event["log_failure"] = object()
        eventText = eventAsText(
            event,
            includeTimestamp=True,
            includeSystem=False
        )
        self.assertIsInstance(eventText, unicode)
        self.assertIn(u"(UNABLE TO OBTAIN TRACEBACK FROM EVENT)", eventText)

        If a traceback cannot be appended, a message indicating this is true
        is appended.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_format.py