def test_criticalLogging(self):
        """
        Critical messages will be written as text to the error stream.
        """
        log = Logger(observer=self.publisher)
        log.info("ignore this")
        log.critical("a critical {message}", message="message")
        self.assertEqual(self.errorStream.getvalue(), u"a critical message\n")

        Critical messages will be written as text to the error stream.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_global.py