def start(self, application):
        """
        Initialize the global logging system for the given application.

        If a custom logger was specified on the command line it will be used.
        If not, and an L{logger.ILogObserver} or legacy L{log.ILogObserver}
        component has been set on C{application}, then it will be used as the
        log observer. Otherwise a log observer will be created based on the
        command line options for built-in loggers (e.g. C{--logfile}).

        @param application: The application on which to check for an
            L{logger.ILogObserver} or legacy L{log.ILogObserver}.
        @type application: L{twisted.python.components.Componentized}
        """
        if self._observerFactory is not None:
            observer = self._observerFactory()
        else:
            observer = application.getComponent(logger.ILogObserver, None)
            if observer is None:
                # If there's no new ILogObserver, try the legacy one
                observer = application.getComponent(log.ILogObserver, None)

        if observer is None:
            observer = self._getLogObserver()
        self._observer = observer

        if logger.ILogObserver.providedBy(self._observer):
            observers = [self._observer]
        elif log.ILogObserver.providedBy(self._observer):
            observers = [logger.LegacyLogObserverWrapper(self._observer)]
        else:
            warnings.warn(
                ("Passing a logger factory which makes log observers which do "
                 "not implement twisted.logger.ILogObserver or "
                 "twisted.python.log.ILogObserver to "
                 "twisted.application.app.AppLogger was deprecated in "
                 "Twisted 16.2. Please use a factory that produces "
                 "twisted.logger.ILogObserver (or the legacy "
                 "twisted.python.log.ILogObserver) implementing objects "
                 "instead."),
                DeprecationWarning,
                stacklevel=2)
            observers = [logger.LegacyLogObserverWrapper(self._observer)]

        logger.globalLogBeginner.beginLoggingTo(observers)
        self._initialLog()

        Initialize the global logging system for the given application.

        If a custom logger was specified on the command line it will be used.
        If not, and an L{logger.ILogObserver} or legacy L{log.ILogObserver}
        component has been set on C{application}, then it will be used as the
        log observer. Otherwise a log observer will be created based on the
        command line options for built-in loggers (e.g. C{--logfile}).

        @param application: The application on which to check for an
            L{logger.ILogObserver} or legacy L{log.ILogObserver}.
        @type application: L{twisted.python.components.Componentized}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/application/app.py