Hi Andy, Your example code is very close to working. The reason the chain didn't proceed as you expected is because the callbacks of the second Deferredcreated in the function print_something were never triggered to run. You need to keep track of what actually triggers a deferred's callback chain to run and what each of the callback functions return. In practice, that second deferred could have been replaced by some asynchronous function that actually returned a deferred, and your code would have worked. The simplest change to make you code work is to change print_something to this: def print_something(x): print "printing: ", x d = defer.Deferred() d.addCallbacks(do_something_inbetween, oops) d.callback("printed something") return d Doing this is kind of redundant though, because in this case it is equivalent to: def print_something(x): print "printing: ", x return do_something_inbetween('printed_something') Another thing you can do instead of creating a new deferred is wrap the call to do_something_inbetween with defer.maybeDeferred. This is convenient because if do_something_inbetween does not return a deferred (which is the case here), then maybeDeferred will do this for you. Here's an example (a slight modification of your original) that includes calling a function that returns a deferred and using maybeDeferred to construct a deferred chain. from twisted.internet import reactor, defer def do_something_inbetween(x): print "doing something else: ", x return "done something else" def print_something(x): print "printing: ", x d = defer.maybeDeferred(do_something_inbetween,'printed something') d.addCallbacks(finish_up, oops) return d def finish_up(x): print "finishing up with: ", x def oops(e): print str(e) def go(): print "constructed deferred" d = defer.Deferred() return d def extra(r): print 'extra', r return r d = go() d.addCallback(print_something) d.addCallback(extra) d.addErrback(oops) reactor.callLater(1, d.callback, "hello mr") reactor.callLater(5, reactor.stop) print "setup everything" reactor.run() Hope that helps. -Dorian On Wed, Jun 18, 2008 at 11:39 PM, Andy Hird <andyhird@gmail.com> wrote:

Hi all,

I've been trying to get a really basic example of chaining deferreds going with no luck. I was hoping someone could give me some help and tell me where I'm going wrong.

I've attached the basic code to this message.

I'd expect output to be:

constructed deferred setup everything printing: hello mr doing something else: printed something finishing up with: done something else

But instead the callback chain stops after print_something() has been called (do_something inbetween and finish_up are never called).

Thanks for any help in advance!

Andy Hird

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