def test_endpointConnectingCancelled(self):
        """
        Calling L{Deferred.cancel} on the L{Deferred} returned from
        L{IStreamClientEndpoint.connect} is errbacked with an expected
        L{ConnectingCancelledError} exception.
        """
        mreactor = MemoryReactor()

        clientFactory = object()

        ep, ignoredArgs, address = self.createClientEndpoint(
            mreactor, clientFactory)

        d = ep.connect(clientFactory)

        receivedFailures = []

        def checkFailure(f):
            receivedFailures.append(f)

        d.addErrback(checkFailure)

        d.cancel()
        # When canceled, the connector will immediately notify its factory that
        # the connection attempt has failed due to a UserError.
        attemptFactory = self.retrieveConnectedFactory(mreactor)
        attemptFactory.clientConnectionFailed(None, Failure(error.UserError()))
        # This should be a feature of MemoryReactor: <http://tm.tl/5630>.

        self.assertEqual(len(receivedFailures), 1)

        failure = receivedFailures[0]

        self.assertIsInstance(failure.value, error.ConnectingCancelledError)
        self.assertEqual(failure.value.address, address)

        Calling L{Deferred.cancel} on the L{Deferred} returned from
        L{IStreamClientEndpoint.connect} is errbacked with an expected
        L{ConnectingCancelledError} exception.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_endpoints.py