def test_triggersRunOnce(self):
        """
        A trigger should only be called on the first call to
        L{_ThreePhaseEvent.fireEvent}.
        """
        events = []
        self.event.addTrigger('before', events.append, 'before')
        self.event.addTrigger('during', events.append, 'during')
        self.event.addTrigger('after', events.append, 'after')
        self.event.fireEvent()
        self.event.fireEvent()
        self.assertEqual(events, ['before', 'during', 'after'])

        A trigger should only be called on the first call to
        L{_ThreePhaseEvent.fireEvent}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_internet.py