Observer pattern / Event oriented programming in Python

I recently decided that in one portion of Ancho's code, it would make sense to use an event-oriented paradigm. I'm familiar with the Observer pattern from the Gang of Four (Design Patterns) book, and I've used event models in things like GUI programming and Javascript. The general idea is that instead of having your code constantly check for some condition, you can tell the event system "let me know when this happens," which can be much more efficient.

Event programming can be hard to wrap your head around at first, but this is in a part of Ancho that casual users probably won't see much. I think it's worth the added complexity.

In Python, there is often only one obvious way to do something. But I've never done event programming in Python before, and initial searches did not yield a single obvious preferred way. So, I'm documenting my search here in hopes that it saves others some effort in answering the same question.

Narrowing the Search

There are some sub-categories of event-based programming:

  1. Networked message passing, job queues, etc. This would include things like Celery on top of an underlying task queue. (Actually seems like a good summary of all the queue systems out there.)
  2. Responding to events from the network, dealing with concurrency and non-blocking I/O. This would include things like Twisted, Eventlet, GEvent, and the like.
  3. Libraries for handling in-process event propagation without regard for networking or I/O. Mostly these are about performance, concurrency issues prevention of memory leaks. These can be synchronous (calls to event handlers must return before the next handler is called) or asynchronous (calls to event handlers execute in their own threads and may be executing simultaneously.) For my purposes, I had planned to use synchronous calls.

For my purposes, I am exclusively interested in the last of the above categories.

Found, but Rejected

  • Python library's signal module. This is really more for dealing with BSD-style signals, which are interruptions from outside the current process, like what happens when you hit Ctrl-C.
  • PyDispatcher. This software was last updated in 2011, which is either because it's really stable and perfect (not likely) or because it's no longer actively used or maintained. Since I'm looking for something that is actively under development, this means that PyDispatcher is out. Besides, Django stopped using PyDispatcher several years ago, in favor of a modified version that was roughly 10 times faster.
  • Michael Foord's Event Hook. This is more of a recipe than an actual piece of code. I'm looking for something that's being maintained.
  • Circuits. This appears to be more heavyweight than what I'm looking for. It is described as a "component architecture" and includes a web server and networking i/o components, which would put it more in my second category.
  • Py-Notify. Last release was in 2008. The author's benchmarking, however, is of interest: compared with PyGTK (a C-based implementation) PyNotify is considerably slower. That's no surprise, but I might use a similar benchmarking method to compare the libraries that seem like viable candidates.
  • Observed. Interesting, but it is more language-level in that it sends events for all method calls.  You don't seem have the level of control to create your own events. Not quite what I was looking for.
  • PySignals. Based on the django.dispatch code that replaced PyDispatcher; but apparently not touched since 2010.


These are all close enough to what I'm looking for for a fuller evaluation.

  • zope.event. Updated in 2014, and seems to be reguarly developed. I'm not sure if it has any dependencies on the rest of Zope; if so, that's a big negative. I have also read that it does not support arguments to be passed through to handlers.  That seems like a potential downside, depending on how my usage patterns shake out.
  • Django's signals implementation. Like Zope.Event, it's probably integrated somewhat with other code I don't need, but it's well-tested in production environments.
  • PyPubSub. Last updated in February 2014, so that's fairly recent. Support activity seems weak.  I don't see many references to it online.  I'll try it anyway, because its implementation is a little different (publish/subscribe with a central dispatcher) than the others.
  • Events. Bills itself as "Python event handling the C# style." The fact that it's C# doesn't do anything for me, but I do like that it's well-documented, has frequent commit activity, and has explicit support for python 2.7, 3.3 and 3.4. 
  • Smokesignal. This seems to be an indirect descendant of Django's signals, as PySignals was, except that this is still actively maintained. As of this writing the last commit was 11 days ago.

So, what I plan to do is to read up on these candidates; develop a test workload similar to my use case that can be implemented on each of them; benchmark them for performance; and report back on their overall fitness and ease of use. My initial guess is that Zope.Event and Django.Dispatch will not be easy fits, because of their connections to those other frameworks... but we'll see.

The heat will be on!