def test_startUsesApplicationLogObserver(self):
        """
        When the L{ILogObserver} component is available on the application,
        that object will be used as the log observer instead of constructing a
        new one.
        """
        application = Componentized()
        observer = self._makeObserver()
        application.setComponent(ILogObserver, observer)
        logger = app.AppLogger({})
        logger.start(application)
        self._checkObserver(observer)

        When the L{ILogObserver} component is available on the application,
        that object will be used as the log observer instead of constructing a
        new one.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_twistd.py