Registry is a quite common design pattern which I recently needed to implement and I found Python class decorators very useful so I thought I’d write about it. :–)
In the registry pattern we have a global object we call the registry or registrar which contains references to shared objects and it’s the only entity which can access those.
In my case, I had a plugin style architecture in which different plugins might be dynamically added without any configuration. The first implementation I made used a metaclass to add the plugin class to the registry:
[gist]https://gist.github.com/771884[/gist]
That worked, at least for the beginning. But later I found a limitation: plugins needed the Plugin metaclass, so I couldn’t use another metaclass on them if I needed to. And at some point I did feel the need to do that. I wanted the plugins to use the Singleton metaclass (to implement the Singleton pattern) but then I couldn’t use the Plugin metaclass because only one metaclass can be used (and I didn’t want to create a SingletonPlugin metaclass). And class decorators saved the day:
[gist]https://gist.github.com/771888[/gist]
Class decorators are supported only in Python >= 2.6, but that wasn’t a problem. :–)
:wq