def test_formatUnformattableEventWithUnformattableKey(self):
        """
        Formatting an unformattable event that has an unformattable key.
        """
        event = {
            "log_format": "{evil()}",
            "evil": lambda: 1 / 0,
            Unformattable(): "gurk",
        }
        result = formatEvent(event)
        self.assertIn("MESSAGE LOST: unformattable object logged:", result)
        self.assertIn("Recoverable data:", result)
        self.assertIn("Exception during formatting:", result)

        Formatting an unformattable event that has an unformattable key.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_format.py