Bob Ippolito (@etrepum) on Haskell, Python, Erlang, JavaScript, etc.

Twisted and foreign event loops


There's a new reactor in Twisted svn: threadedselectreactor. This reactor blocks on select() in a worker thread, so that it is easy to integrate with a foreign event loop without having to completely separate your Twisted code into its own thread. This is awesome, and mostly eliminates the need to have specific reactors for each useful foreign event loop (cfreactor, wxreactor, etc.). Especially when the existing integration reactor sucks -- wxreactor is more or less a worst-possible implementation, and the others aren't a whole lot better.

In order to integrate with foreign event loops, threadedselectreactor sports an extension to the reactor interface: interleave(waker). waker is a callable that takes a callable as an argument. Its job is to call this callable from the thread of the main event loop. So, whatever you're integrating with needs to be able to send messages from an arbitrary thread to the main thread. All of the event loops worth the fairy dust their bits are encoded on are designed to do this easily.

Other than that, the only other thing that needs to be considered is shutdown (if it matters to shut down cleanly). To do that, you can simply replace your "quit" function with an implementation that:

  • Sets up an "after shutdown" reactor system event that will call "quitRightNow" when the reactor is dead
  • Calls reactor.stop() to commence the self destruct sequence

wxPython already has wxCallAfter that works perfectly well as the waker argument for interleave, so you can Twistify a wxPython application in about six lines. Here's a full demo: doc/examples/core/threadedselect/

PyObjC has a PyObjCTools.AppHelper.callAfter that also serves as a too-trivial-to-be-true waker (coincidence? no -- I just added it). You can check out the demo from: doc/examples/core/threadedselect/Cocoa/SimpleWebClient/. The examples in PyObjC's Examples/Twisted/ have also been refactored to use this (instead of cfreactor, which should be considered dead weight).

pygame is more low-level and doesn't provide any conveniences beyond getting and posting events. So your waker should be some function that posts a USEREVENT to pygame, which you need to pick up and do something with. A minimal example of this is here: doc/examples/core/threadedselect/

This reactor should trivially integrate with just about any event loop, including PyQt, GTK, etc. However, the margins of my blog aren't big enough to fit them all.