Hi, I'm trying to get one of my twisted programs to terminate immediately if it can't listen on a port on startup. It's main program is basically the same as in e.g. doc/howto/listings/application/app1.py, i.e (quoting from app1.py): app = twisted.internet.app.Application("daytimer") f = Factory() f.protocol = Daytime app.listenTCP(8813, f) app.run() My program uses a different protocol and factory, but the principle is the same: it calls app.listenTCP before it calls app.run(). Now, if the mainloop isn't running yet, listenTCP doesn't start listening immediately[1] but simply appends the arguments to a list. Then, when app.run() is called and that list is not empty it will call bindPorts before entering the mainloop. bindPorts simply catches the CannotListenError, logs a message and returns immediately even if not all ports have been bound yet. Unfortunately, even if bindPorts returns early, app.run still enters the mainloop. There doesn't seem to be a good way to avoid this. IMO bindPorts should reraise the exception after writing logging the error instead of simply returning. Bernhard [1] I think app.listenTCP doesn't try to listen immediately because this method is also used when creating taps and that should be possible even if the port is currently in use or the user doesn't have permission to listen on that port. -- Intevation GmbH http://intevation.de/ Sketch http://sketch.sourceforge.net/ MapIt! http://www.mapit.de/