def addSystemEventTrigger(self, _phase, _eventType, _f, *args, **kw):
        """See twisted.internet.interfaces.IReactorCore.addSystemEventTrigger.
        """
        assert callable(_f), "%s is not callable" % _f
        if _eventType not in self._eventTriggers:
            self._eventTriggers[_eventType] = _ThreePhaseEvent()
        return (_eventType, self._eventTriggers[_eventType].addTrigger(
            _phase, _f, *args, **kw))
See twisted.internet.interfaces.IReactorCore.addSystemEventTrigger.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/base.py