[Twisted-Python] Passing additional arguments to errback

[Laurens Van Houtven]

Hi,


On Thu, Sep 5, 2013 at 9:24 AM, Maciej Wasilak <wasilak at 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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