[Twisted-Python] Newbie question: can't get getProcessOutput to run from inside a listenTCP service call-back

[Nick Joyce]

On 17 Oct 2009, at 05:15, Read Roberts wrote:

> I have a Python server using twisted which responds to pyAMF calls  
> from
> Flex/Air applications (following Bruce Eckels 5/1/2008 article  
> "Concurrency
> with Python, Twisted, and Flex"). The service is basically as  
> follows below.
> When the client calls a service API, the API needs to make a OS  
> shell call.
> I see that if I call reactor.stop() after the  getProcessOutput call  
> (such
> as within the doLog and doError functions), the call happens and I  
> see the
> output. How can can I retrieve the output without calling  
> reactor.stop()? In
> the example below, the AIR app calls the callback helloWorldTest(),  
> and the
> deferred.addCallback function is never called.
>
> import os
> from twisted.spread import pb
> from  twisted.internet.utils import getProcessOutput
> from twisted.internet import reactor
>
> class FlexInterface(pb.Root):
>    def __init__(self):
>        self.result e
>
>    def doLog(self, result):
>        self.result = "%s" % (result)
>        return result
>
>    def doError(self, failure):
>        self.result = "%s" % ( failure)
>        return failure
>
>    def helloWorldTest(self):
>        deferred = getProcessOutput("echo", ["Hello World"],  
> os.environ)
>        deferred.addErrback(self.doError)
>        deferred.addCallback(self.doLog)
>        while (self.result == None)
>            print "doLog still hasn't been called"
>        return self.result


The while loop will block the event loop, preventing the deferred from  
ever being called. PyAMF will accept a deferred as a return value for  
the service function and will not return a response until the  
deferred's callback is fired.

So, you could re-write the method like:

     def helloWorldTest(self):
         deferred = getProcessOutput("echo", ["Hello World"],  
os.environ)

         deferred.addErrback(self.doError)
         deferred.addCallback(self.doLog)

         return deferred

If the deferred's callback is called then the return value of doLog  
will be used as the response value. If the errback is called, the  
return value of doError will be used as the response, in this case a  
Failure instance. PyAMF converts Failure instances to remoting/ 
messaging error objects automagically, so the faultHandler on the  
flash/air side will be called.

hth,

Nick