Twisted and a command line interface

Question

I have a program where I want to do two things:

1. Interact with a server and respond to events from the server. I am doing this using twisted.

2. Have a command line prompt for the user where he can issue additional commands. I am using the python cmd module for this so far.

There seems to be no other choice than having two threads, as readline only has a blocking interface and needs to handle stuff like auto completion. Twisted on the other hand has to continuously run the reactor.

Now the problem is that it seems very hard to handle Ctrl-C for this. The easy solution would seem to have the command line run in the main thread and just use reactor.callFromThread for every interaction with the rest of the program. This is very easy, as overwriting Cmd.onecmd can do this in a generic way. However when I try to spawn the reactor in a thread with

t = Thread(target=reactor.run)
t.start()

I immediately get an exception

File "/usr/lib/python3.6/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
builtins.ValueError: signal only works in main thread

Everyone using twisted insists that the twisted reactor should run in the main thread, as that would be a better design.

When trying to do it that way and running twisted in the main thread, it will catch the Ctrl-C, exit the reactor and I am stuck with a thread that does not exit, as the call to input() inside cmdloop does not return. I tried searching for a solution to this and how to get out of the input() call, but everyone also insists that a command line interface should run in the main thread.

One potential option I found was to run twisted as the main thread and make the input thread a daemon, so it should exit when the reactor exits, however the daemon flag did not change anything (the thread did not exit when the main thread did). Furthermore this is likely dangerous, as the thread might be doing something important when it is killed.

Is there any way out of this?

Answer 1

Take a look at how invective does this with Twisted and without threads (one way to read the code might be to start at the mainpoint and work your way in).