[Twisted-Python] Re: invoking Deferred synchronously

[Laurie Harper]

Phil Mayers wrote:
> On Tue, 2007-08-21 at 05:09 -0400, Laurie Harper wrote:
>> I'm sure this is trivial enough, but it has me scratching my head... I 
>> need to call a method that returns a Deferred, then block until the 
>> deferred completes (or errback's) and return the final result of the 
>> deferred call.
> 
> You don't want to do that.

Perhaps once I get more used to Twisted's programming model, I'll stop 
tending to ;-)

>> Specifically, something like:
>>
>>      r = None, is_ok = None
>>      def ok(p): r, is_ok = p, True
>>      def fail(e): r, is_ok = e, False
>>
>>      cf = pb.PBClientFactory()
>>      reactor.connectTCP(host, port, cf)
>>      d = cf.login(creds, client=self).addCallbacks(ok, fail)
>>
>>      # BLOCK: wait until d has called 'ok' of 'fail'
>>
>>      if is_ok:
>>          return r
>>      else:
>>          raise r
>>
>> If it helps, I'm trying to create a web application (using Nevow) that 
>> connects to another Twister service through the Persistence Broker. I'm 
>> trying to implement authentication, and so need a way to wait for the PB 
>> connection to succeed/fail so I can send the appropriate response to the 
>> HTTP request.
> 
> You certainly don't need to do that in a Nevow app. Just do this (I'm
> assuming this is in a data or render method):
> 
> cf = pb.PBClientFactory()
> reactor.connectTCP(host, port, cf)
> return cf.login(creds, client=self)
> 
> Nevow will see you've returned a deferred and wait for it. Much of
> Twisted will do the same. You NEVER want to "wait" for a deferred.

Wow, nice; it never occurred to me to try that :-) That isn't quite what 
I'd need, since I want to retrieve the IPresence returned from login() 
and store it in the web server (so it can be used to service subsequent 
HTTP requests). However, from the experimentation I did earlier with 
inlineCallbacks and the like I think I know how to wrap a deferred up 
and do what I need.

Thanks for the pointer, I'll give it a try.

L.