[Twisted-Python] Issue with calling back deferreds from threads

Bob Ippolito bob at redivi.com
Wed Jul 14 19:31:09 EDT 2004

On Jul 14, 2004, at 6:27 PM, Ken Kinder wrote:

> Introduced in one of the recent versions of Twisted was a patch  
> described here. I'm cc'ing itamar because he created the patch:
> http://twistedmatrix.com/pipermail/twisted-commits/2003-November/ 
> 010059.html
> This makes sure you can't callback with Deferred objects. However,  
> some of my code depends on doing just that because of the blocking DNS  
> issue:
> def mymethod(*args):
>    def go(*args):
>       return xmlproxy.callRemote(*args)
>    twisted.internet.threads.deferToThread(go)
> As you can see, I'm deferring something that should not be blocking to  
> a thread. The purpose for doing so is that sometimes resolving the  
> hostname can take a while for this call. (Unrelated document issue  
> with Twisted: DNS is blocking.) So, in this case, it is desirable  
> (although hackish) to defer the creation of a deferred object to a  
> thread, and itamar's patch prevents this process.

Isn't there a quick way to turn on Twisted's DNS client, so that DNS is  

> One option I have is to make go() blocking, but that will hold a  
> thread open for longer than is necessary. Another option is to comment  
> out the assertion, but that's probably not a good idea. Until the DNS  
> blocking issue is resolved, I'm not sure how to solve this one.
> Suggestions?

Yeah, use reactor.callFromThread.

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