def test_criticalLoggingStops(self):
        """
        Once logging has begun with C{beginLoggingTo}, critical messages are no
        longer written to the output stream.
        """
        log = Logger(observer=self.publisher)
        self.beginner.beginLoggingTo(())
        log.critical("another critical message")
        self.assertEqual(self.errorStream.getvalue(), u"")

        Once logging has begun with C{beginLoggingTo}, critical messages are no
        longer written to the output stream.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_global.py