def test_cancelFiredOnOneErrbackDeferredList(self):
        """
        When a L{defer.DeferredList} has fired because one L{defer.Deferred} in
        the list fired with a failure result, the cancellation will do
        nothing instead of cancelling the rest of the L{defer.Deferred}s.
        """
        deferredOne = defer.Deferred()
        deferredTwo = defer.Deferred()
        deferredList = defer.DeferredList([deferredOne, deferredTwo],
                                          fireOnOneErrback=True)
        deferredOne.errback(GenericError("test"))
        deferredList.cancel()
        self.assertNoResult(deferredTwo)
        self.failureResultOf(deferredOne, GenericError)
        self.failureResultOf(deferredList, defer.FirstError)

        When a L{defer.DeferredList} has fired because one L{defer.Deferred} in
        the list fired with a failure result, the cancellation will do
        nothing instead of cancelling the rest of the L{defer.Deferred}s.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_defer.py