def _brokenValueTest(self, detail):
        """
        Construct a L{Failure} with an exception that raises an exception from
        its C{__str__} method and then call C{getTraceback} with the specified
        detail and verify that it returns a string.
        """
        x = BrokenStr()
        f = failure.Failure(x)
        traceback = f.getTraceback(detail=detail)
        self.assertIsInstance(traceback, str)

        Construct a L{Failure} with an exception that raises an exception from
        its C{__str__} method and then call C{getTraceback} with the specified
        detail and verify that it returns a string.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_failure.py