def test_resultOfDeferredResultOfDeferredOfFiredDeferredCalled(self):
        """
        Given three Deferreds, one chained to the next chained to the next,
        callbacks on the middle Deferred which are added after the chain is
        created are called once the last Deferred fires.

        This is more of a regression-style test.  It doesn't exercise any
        particular code path through the current implementation of Deferred, but
        it does exercise a broken codepath through one of the variations of the
        implementation proposed as a resolution to ticket #411.
        """
        first = defer.Deferred()
        second = defer.Deferred()
        third = defer.Deferred()
        first.addCallback(lambda ignored: second)
        second.addCallback(lambda ignored: third)
        second.callback(None)
        first.callback(None)
        third.callback(None)
        L = []
        second.addCallback(L.append)
        self.assertEqual(L, [None])

        Given three Deferreds, one chained to the next chained to the next,
        callbacks on the middle Deferred which are added after the chain is
        created are called once the last Deferred fires.

        This is more of a regression-style test.  It doesn't exercise any
        particular code path through the current implementation of Deferred, but
        it does exercise a broken codepath through one of the variations of the
        implementation proposed as a resolution to ticket #411.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py