http://krondo.com/?p=1333

twisted_exercise_countdown.py

class Countdown(object):

    n_counters = 0 # class-level variable: shared between all instances

    def __init__(self, counter=5, delay=1):
        self.counter = counter
        self.delay = delay
        Countdown.n_counters += 1

    def count(self):
        if self.counter == 0:
            Countdown.n_counters -= 1
            print "A countdown has finished, %s remaining" % Countdown.n_counters
            if not Countdown.n_counters:
                reactor.stop()
        else:
            print self.counter, '...'
            self.counter -= 1
            reactor.callLater(self.delay, self.count)

from twisted.internet import reactor

reactor.callWhenRunning(Countdown(5).count)
reactor.callWhenRunning(Countdown(2, 5).count)
reactor.callWhenRunning(Countdown(10, 0.5).count)

print 'Start!'
reactor.run()
print 'Stop!'