def test_subsequentBeforeTriggerFiresPriorBeforeDeferred(self):
        """
        If a trigger added to the C{'before'} phase of an event calls back a
        L{Deferred} returned by an earlier trigger in the C{'before'} phase of
        the same event, the remaining C{'before'} triggers for that event
        should be run and any further L{Deferred}s waited on before proceeding
        to the C{'during'} events.
        """
        eventType = 'test'
        events = []
        firstDeferred = Deferred()
        secondDeferred = Deferred()
        def firstBeforeTrigger():
            return firstDeferred
        def secondBeforeTrigger():
            firstDeferred.callback(None)
        def thirdBeforeTrigger():
            events.append('before')
            return secondDeferred
        def duringTrigger():
            events.append('during')
        self.addTrigger('before', eventType, firstBeforeTrigger)
        self.addTrigger('before', eventType, secondBeforeTrigger)
        self.addTrigger('before', eventType, thirdBeforeTrigger)
        self.addTrigger('during', eventType, duringTrigger)
        self.assertEqual(events, [])
        reactor.fireSystemEvent(eventType)
        self.assertEqual(events, ['before'])
        secondDeferred.callback(None)
        self.assertEqual(events, ['before', 'during'])

        If a trigger added to the C{'before'} phase of an event calls back a
        L{Deferred} returned by an earlier trigger in the C{'before'} phase of
        the same event, the remaining C{'before'} triggers for that event
        should be run and any further L{Deferred}s waited on before proceeding
        to the C{'during'} events.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_internet.py