defer-cancel-11.py

from twisted.internet.defer import Deferred

def send_poem(d):
    print('Sending poem')
    d.callback('Once upon a midnight dreary')

def get_poem():
    """Return a poem 5 seconds later."""

    def canceler(d):
        # They don't want the poem anymore, so cancel the delayed call
        delayed_call.cancel()

        # At this point we have three choices:
        #   1. Do nothing, and the deferred will fire the errback
        #      chain with CancelledError.
        #   2. Fire the errback chain with a different error.
        #   3. Fire the callback chain with an alternative result.

    d = Deferred(canceler)

    from twisted.internet import reactor
    delayed_call = reactor.callLater(5, send_poem, d)

    return d


def got_poem(poem):
    print('I got a poem:', poem)

def poem_error(err):
    print('get_poem failed:', err)

def main():
    from twisted.internet import reactor
    reactor.callLater(10, reactor.stop) # stop the reactor in 10 seconds

    d = get_poem()
    d.addCallbacks(got_poem, poem_error)

    reactor.callLater(2, d.cancel) # cancel after 2 seconds

    reactor.run()

main()