Steve Freitas wrote:
Hi all,
I've got an XML-RPC app that has to grab some data from a SQL database, then depending on the results, make some more SQL calls, make some more decisions, then finally return a result. So, I'm hip-deep in Deferreds. So, here's the idiom I've come up with, and I'm running into a limitation with it, namely that I can't seem to add callbacks to a Deferred after it has started running. Here's what I'm doing in abbreviated form, and it's an idiom I'd like to stick with, if possible:
This wasn't spectacularly obvious to me the first time either, but I think it's covered somewhere in the deferred execution howto; in your callback/errback, return a new deferred and the processing of the deferred chain will pause until that new deferred fires. Here's an example of code that does that... df = self.shouldRetrieve( urlRecord ) def ifWeShould( judgement, urlRecord ): if judgement: df = urlRecord.retrieve( self.timeout, self.userAgent ) df.addCallback( self.onSuccess, urlRecord = urlRecord ) df.addErrback( self.onFailure, urlRecord = urlRecord ) return df else: return self.onFailure( failure.Failure( RobotDenied(urlRecord)), urlRecord = urlRecord ) df.addCallback( ifWeShould, urlRecord=urlRecord ) In the one branch, we return the result of a call, in the other, we return a new Deferred. Note that the new deferred shows up between the current callback and any Deferred higher up in the chain, so the Deferred's higher up the chain will get the result of the new Deferred. If that's not desirable, you can define a handler for the new deferred that returns the original result. Hope I understood the question properly and that this helped, Mike ________________________________________________ Mike C. Fletcher Designer, VR Plumber, Coder http://www.vrplumber.com http://blog.vrplumber.com