def test_removeObserverNotRegistered(self):
        """
        L{LogPublisher.removeObserver} removes an observer that is not
        registered.
        """
        o1 = lambda e: None
        o2 = lambda e: None
        o3 = lambda e: None

        publisher = LogPublisher(o1, o2)
        publisher.removeObserver(o3)
        self.assertEqual(set((o1, o2)), set(publisher._observers))

        L{LogPublisher.removeObserver} removes an observer that is not
        registered.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/test/test_observer.py