def test_cancelDeferredListWithFireOnOneCallback(self):
        """
        When cancelling an unfired L{defer.DeferredList} with the flag
        C{fireOnOneCallback} set, cancel every L{defer.Deferred} in the list.
        """
        deferredOne = defer.Deferred()
        deferredTwo = defer.Deferred()
        deferredList = defer.DeferredList([deferredOne, deferredTwo],
                                          fireOnOneCallback=True)
        deferredList.cancel()
        self.failureResultOf(deferredOne, defer.CancelledError)
        self.failureResultOf(deferredTwo, defer.CancelledError)

        When cancelling an unfired L{defer.DeferredList} with the flag
        C{fireOnOneCallback} set, cancel every L{defer.Deferred} in the list.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py