Question

I’m currently repeating a task in a for loop inside a callback using Twisted, but would like the reactor to break the loop in the callback (one) if the user issues a KeyboardInterrupt via Ctrl-C. From what I have tested, the reactor only stops or processes interrupts at the end of the callback.

Is there any way of sending a KeyboardInterrupt to the callback or the error handler in the middle of the callback run?

Cheers,

Chris

#!/usr/bin/env python

from twisted.internet import reactor, defer


def one(result):
    print "Start one()"
    for i in xrange(10000):
        print i
    print "End one()"
    reactor.stop()


def oneErrorHandler(failure):
    print failure
    print "INTERRUPTING one()"
    reactor.stop()    


if __name__ == '__main__':

    d = defer.Deferred()
    d.addCallback(one)
    d.addErrback(oneErrorHandler)
    reactor.callLater(1, d.callback, 'result')

    print "STARTING REACTOR..."
    try:
        reactor.run()
    except KeyboardInterrupt:
        print "Interrupted by keyboard. Exiting."
        reactor.stop()

Answer 1

This is intentional to avoid (semi-)preemption, since Twisted is a cooperative multitasking system. Ctrl-C is handled in Python with a SIGINT handler installed by the interpreter at startup. The handler sets a flag when it is invoked. After each byte code is executed, the interpreter checks the flag. If it is set, KeyboardInterrupt is raised at that point.

The reactor installs its own SIGINT handler. This replaces the behavior of the interpreter’s handler. The reactor’s handler initiates reactor shutdown. Since it doesn’t raise an exception, it doesn’t interrupt whatever code is running. The loop (or whatever) gets to finish, and when control is returned to the reactor, shutdown proceeds.

If you’d rather have Ctrl-C (ie SIGINT) raise KeyboardInterrupt, then you can just restore Python’s SIGINT handler using the signal module:

signal.signal(signal.SIGINT, signal.default_int_handler)

Note, however, that if you send a SIGINT while code from Twisted is running, rather than your own application code, the behavior is undefined, as Twisted does not expect to be interrupted by KeyboardInterrupt.