def test_formatMessage(self):
        """
        L{_ErrorFormatter.formatError} should return the return value of
        C{formatMessage} if it is supplied.
        """
        formatCalls = []
        def formatMessage(errorCode):
            formatCalls.append(errorCode)
            return self.probeMessage
        formatter = _ErrorFormatter(
            None, formatMessage, {self.probeErrorCode: 'wrong message'})
        message = formatter.formatError(self.probeErrorCode)
        self.assertEqual(message, self.probeMessage)
        self.assertEqual(formatCalls, [self.probeErrorCode])

        L{_ErrorFormatter.formatError} should return the return value of
        C{formatMessage} if it is supplied.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_strerror.py