TND dataview and metaclass magic in python - Maarten ter Horst¶

Tags: django, pun

(Presentation at the Dutch Django/web meeting)

Metaclass magic feels like magic, but it isn’t real magic. Metaclasses are ideal for frameworks and code you want to reuse.

They make web applications that have to feel like desktop apps. Very fast and responsive. TND Dataview (note: a non-opensource project he used as example for metaclasses) are fully ajax based CRUD views for Django that are very fast.

A dataview is just a view, but it subclasses Dataview, which fires up all sorts of metaclass magic. This is not the Meta class on your model, but a metaclass is a python class that itself creates classes. Normally a class is generated behind the scenes with type(), but with the proper invocation, a metaclass takes over type()’s work. For that, the metaclass should have a __new__() next to the common __init__().

You use a metaclass by specifying an __metaclass__ attribute on one of your classes that points at a metaclass.

A metaclass’s __new__() method can do do everything python can do. The possibilities are endless. You can do extra logging, you can look at the attributes of the class and do special things to them, etc. (Note by Reinout: django itself uses it for models, for instance).

An example: dataview’s metaclass creates the necessary URLs for your view. It creates filters based on your data. It uses declarations (=attributes on the view and on the class

A nice tutorial: http://www.ibm.com/developerworks/linux/library/l-pymeta/index.html

Dataview itself is not open source. It is not good enough yet. They could make it available. But… the main goal of his presentation is not dataview as such, but to get us enthousiastic about metaclasses.