def test_connectionFailed(self):
        """
        If a connection cannot be established, the L{Deferred} returned by
        L{SSHCommandClientEndpoint.connect} fires with a L{Failure}
        representing the reason for the connection setup failure.
        """
        endpoint = SSHCommandClientEndpoint.newConnection(
            self.reactor, b"/bin/ls -l", b"dummy user",
            self.hostname, self.port, knownHosts=self.knownHosts,
            ui=FixedResponseUI(False))
        factory = Factory()
        factory.protocol = Protocol
        d = endpoint.connect(factory)

        factory = self.reactor.tcpClients[0][2]
        factory.clientConnectionFailed(None, Failure(ConnectionRefusedError()))

        self.failureResultOf(d).trap(ConnectionRefusedError)

        If a connection cannot be established, the L{Deferred} returned by
        L{SSHCommandClientEndpoint.connect} fires with a L{Failure}
        representing the reason for the connection setup failure.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/test/test_endpoints.py