Hi,
On Thu, Sep 5, 2013 at 9:24 AM, Maciej Wasilak <wasi...@gmail.com> wrote: > Thank you for your answer. I'm explicitly interested in the following > combination: > > > .addCallback(cb).addErrback(eb) > > If I understand correctly errback "eb" catches errors from both callback > "cb", and from agent.request (agent.request errors pass through default > empty errback that re-raises them). > Yes, that's exactly what it does :) The difference being that with .addCallbacks(cb, eb), errors in cb would not be caught by eb. > > Also, if you want to do scraping with Twisted, consider looking at > Scrapy, a fully-featured web scraper that uses Twisted internally. > > I'm working on CoAP protocol which runs on top of UDP. My actual code is > request processing (but the problem is the same as in my previous post): > > _________________________________________________ > def processRequest(self, request): > (...) > d = defer.succeed(request) > d.addCallback(self.processBlock1) > d.addCallback(self.dispatchRequest) > def dispatchRequest(self, request): > (...) > resource = self.endpoint.getResourceFor(request) > if resource is None: > response = Message(code=NOT_FOUND, payload='Resource not > found!') > self.respond(response, request) > return > try: > d = resource.render(request) > except iot.error.UnallowedMethod: > response = Message(code=METHOD_NOT_ALLOWED, payload='Method > not allowed!') > self.respond(response, request) > return > except iot.error.UnsupportedMethod: > response = Message(code=NOT_IMPLEMENTED, payload='Method not > implemented!') > self.respond(response, request) > return > delayed_ack = reactor.callLater(EMPTY_ACK_DELAY, self.sendEmptyAck > , request) > d.addCallback(self.respond, request, delayed_ack) > return d > > __________________________________________________ > > I would like to rewrite it to: > > __________________________________________________ > def processRequest(self, request): > (...) > d = defer.succeed(request) > d.addCallback(self.processBlock1) > d.addCallback(self.dispatchRequest > ).addErrback(self.handleRequestError) > > def dispatchRequest(self, request): > (...) > resource = self.endpoint.getResourceFor(request) > if resource is None: > raise NoResource() > try: > d = resource.render(request) > except iot.error.UnallowedMethod: #Explicit re-rise for this > example only > raise UnallowedMethod() > except iot.error.UnsupportedMethod: > raise UnsupportedMethod() > d.addCallback(self.respond, request) > return d > > > def handleRequestErrors(self, failure, request???) > # handle exceptions, send error response to client > > _______________________________________________ > > I would like to handle Exceptions in handleRequestErrors(). However > handleRequestErrors() has to send a response to the client, so it needs the > request, and I don't know how to pass it from inside dispatchRequest(). > You don't have to do it from in there. You can do .addErrback(handleErrors, request), since it's all the same request object, right? > I see two possibilities: > 1. Pass request inside Failure object > 2. Leave the original code (process errors inside callback) > Both of those would work, but see above :) Two random pieces of code review: 1. It seems self.endpoint is a t.w.s.Site object. That's kind of confusing, since twisted has an "endpoint" concept that's one step removed from a Site (endpoints connect or listen with factories, a Site is a factory). You might want to reconsider that name :) 2. It's kind of strange to start with defer.succeed() and then start making a callback chain IMHO, but it's not wrong, really. cheers lvh
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