def test_singleFrame(self):
        """
        A C{_Traceback} object constructed with a single frame should be able
        to be passed to L{traceback.extract_tb}, and we should get a singleton
        list containing a (filename, lineno, methodname, line) tuple.
        """
        tb = failure._Traceback([], [['method', 'filename.py', 123, {}, {}]])
        # Note that we don't need to test that extract_tb correctly extracts
        # the line's contents. In this case, since filename.py doesn't exist,
        # it will just use None.
        self.assertEqual(traceback.extract_tb(tb),
                         [_tb('filename.py', 123, 'method', None)])

        A C{_Traceback} object constructed with a single frame should be able
        to be passed to L{traceback.extract_tb}, and we should get a singleton
        list containing a (filename, lineno, methodname, line) tuple.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_failure.py