def test_emptyErrorTab(self):
        """
        L{_ErrorFormatter.formatError} should use L{os.strerror} to format
        error messages if it is constructed with only an error tab which does
        not contain the error code it is called with.
        """
        error = 1
        # Sanity check
        self.assertNotEqual(self.probeErrorCode, error)
        formatter = _ErrorFormatter(None, None, {error: 'wrong message'})
        message = formatter.formatError(self.probeErrorCode)
        self.assertEqual(message, os.strerror(self.probeErrorCode))

        L{_ErrorFormatter.formatError} should use L{os.strerror} to format
        error messages if it is constructed with only an error tab which does
        not contain the error code it is called with.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_strerror.py