def test_cancelDeferredListWithFireOnOneCallbackAndDeferredCallback(self):
        """
        When cancelling an unfired L{defer.DeferredList} with the flag
        C{fireOnOneCallback} set, if one of the L{defer.Deferred} callbacks
        in its canceller, the L{defer.DeferredList} will callback with the
        result and the index of the L{defer.Deferred} in a C{tuple}.
        """
        deferredOne = defer.Deferred(fakeCallbackCanceller)
        deferredTwo = defer.Deferred()
        deferredList = defer.DeferredList([deferredOne, deferredTwo],
                                          fireOnOneCallback=True)
        deferredList.cancel()
        self.failureResultOf(deferredTwo, defer.CancelledError)
        result = self.successResultOf(deferredList)
        self.assertEqual(result, ("Callback Result", 0))

        When cancelling an unfired L{defer.DeferredList} with the flag
        C{fireOnOneCallback} set, if one of the L{defer.Deferred} callbacks
        in its canceller, the L{defer.DeferredList} will callback with the
        result and the index of the L{defer.Deferred} in a C{tuple}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py