How to pass connection success or failure to the caller in the Twisted framework?

Question

After working through some of the basic tutorials, I want my TCP/UDP client to exit with a code indicating whether it connected or not. The right way to return an exit code in Twisted is:

point = TCP4ClientEndpoint(reactor, "localhost", 1234)
d = connectProtocol(point, ClientProtocol())
reactor.run()
sys.exit(0)

Then, when the process terminates, it will exit with code 0 to indicate a normal termination. If the client instead times out instead of successfully connecting, how should it pass a value back to that can then be passed to sys.exit instead of the constant 0?

Answer 1

Determining whether the TCP connection succeeded or failed is accomplished by attending to the result of the Deferred:

d = connectProtocol(point, ClientProtocol())
d.addCallbacks(
    connected,
    did_not_connect,
)

With appropriate implementations of connected and did_not_connect, you should be able to pass a suitable value to a subsequent sys.exit call.

For example,

class Main(object):
    result = 1

    def connected(self, passthrough):
        self.result = 0
        return passthrough

    def did_not_connect(self, passthrough):
        self.result = 2
        return passthrough

    def exit(self):
        sys.exit(self.result)

main = Main()
d = connectProtocol(point, ClientProtocol())
d.addCallbacks(
    main.connected,
    main.did_not_connect,
)
reactor.run()
main.exit()