I first came across Twisted Python and Deferreds years ago, and it was my first introduction to the world of asynchronous programming. It was a little bit difficult to wrap my head around the intricate braid of callbacks and errbacks, but once I realised they mapped directly onto sequential statements and try/except in synchronous code, I got a lot done.

In the intervening years, I've seen a lot of projects re-implementing asynchronous programming (A perfectly valid idea, Twisted certainly has enough backwards-compatibility warts to warrant an occasional rebuilding), but the early attempts used bare callbacks for everything, which I found frustrating because I felt Deferreds were clearly a better system. Later attempts introduced "Futures" or "Promises" which were slightly better than bare callbacks but did silly things like pass the same initial result to every registered callback instead of being composable. These almost-but-not-quite-entirely-unlike-Deferreds were even more infuriating, in a "somebody is wrong on the Internet" kind of way.

I'm really glad to see Guido picking the best ideas out of Twisted's Deferreds and trying to make them accessible to a professional-developer audience. If your language/runtime doesn't support some kind of co-routines, then your only hope for dealing with asynchronous code to tame callbacks, and Deferreds are the best model I've come across for doing that.

(Guido mentions he won't be using Deferreds or Deferred-analogues in his Tulip async framework, for undisclosed reasons. I'm betting those reasons are that modern Python does have workable co-routines, so callbacks and callback-taming aren't necessary.)