Can't block for a Deferred in Twisted
Despite the existence of the promising
deferredGenerator and the newer
inlineCallbacks, it appears there is no way to block while waiting for a Deferred. Brian Granger described the problem on the Twisted mailing list:
I have a function that returns a Deferred. I need to have the result of this Deferred returned in a (apparently) blocking/synchronous manner:glyph provided the succinct answer (as well as an interesting commentary on using Twisted the wrong way).def myfuncBlocking(): d = myfuncReturnsDeferred() ... result = return resultI need to be able to call this function like:result = myfuncBlocking()The question is how to get the result out of the Deferred() and make it *look* like myfuncBlocking() has blocked.
This issue has been discussed repeatedly - long story short, it's just a bad idea.
Hmmm, maybe learning Twisted will be harder than I thought.
Update 2008-10-20: Marcin Kasperski wrote a good example comparing raw deferreds, deferred generators, and inline callbacks.
- Twisted web POST example w/ JSON — posted 2010-08-25
- Quick notes on trying the Twisted websocket branch example — posted 2010-05-24
- Running a Twisted Perspective Broker example with twistd — posted 2008-10-27
- Twisted links — posted 2008-10-21
- Running functions periodically using Twisted's LoopingCall — posted 2008-10-14
You can easily use Queue.Queue to turn a Defrred call into a blocking call:
def block_on(d, timeout=None): q = Queue() d.addBoth(q.put) try: ret = q.get(timeout is not None, timeout) except Empty: raise Timeout if isinstance(ret, Failure): ret.raiseException() else: return ret
make sure you never make blocking calls from the reactor thread though.