def test_endpointListenFailure(self):
        """
        When an endpoint tries to listen on an already listening port, a
        C{CannotListenError} failure is errbacked.
        """
        factory = object()
        exception = error.CannotListenError('', 80, factory)
        mreactor = RaisingMemoryReactor(listenException=exception)

        ep, ignoredArgs, ignoredDest = self.createServerEndpoint(
            mreactor, factory)

        d = ep.listen(object())

        receivedExceptions = []

        def checkFailure(f):
            receivedExceptions.append(f.value)

        d.addErrback(checkFailure)

        self.assertEqual(receivedExceptions, [exception])

        When an endpoint tries to listen on an already listening port, a
        C{CannotListenError} failure is errbacked.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/test/test_endpoints.py