[PEAK] Twisted Deferred as EventSources & Generators

Chad Rosenberg chad at idrankwhat.net
Wed Jan 28 15:27:15 EST 2004

I've run into the problem where I need to wait for the results of a 
Deferred while in _run of an AbstractCommand.  I noticed the following 
doesn't work:

     eventLoop = binding.Obtain(IEventSource)

     def doSomeWork(self):
         d = self.somePBCall()
         result = self.eventLoop.runUntil(d)

The above gives me:

     ("Can't adapt", <Deferred at 0x1038af8>, <class 

So I took a look at the adapter DeferredAsEventSource and noticed that 
it only adapts a Deferred to ITaskSwitch, and not IEventSource (as the 
name would seem to imply).   So, I suppose my question is: how does one 
obtain the results of a deferred outside a generator/event.threaded 

Alternatively, as workaround I suppose that I could do something like 
the following?:

     def doSomeWork(self):
         t = self.getPBData()

     def getPBData(self)
         d = self.somePBCall()
         yield d; results = events.resume()
   getPBData = events.threaded(getPBData)

But, even if I were to do the above I'm not quite sure how I could pass 
the results of getPBData back up to doSomeWork.  I guess I could pass 
in a event source Value, but this is turning into a lot of extra code 
just to get the value of a deferred.

I was wondering, if something like the following wouldn't be useful:

     def doSomeWork(self):
         d = self.getPBData()
         yield d; result = events.resume()

         if result == 'something bad':
             raise Exception, 'bad'

         yield events.exit(result)
     doSomeWork = events.waitFor(doSomeWork)

     def _run(self):
             print self.doSomeWork()
         except Exception, reason:
             print reason

Where event.waitFor would wrap a Thread and provide a blocking call.  
That way, as far as the caller is concerned, doSomeWork looks like any 
other blocking method with a return value, and doSomeWork gets to use 
events nice and cleanly.  Or is there already a way to do this that I 
have missed?



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