Friday, March 6, 2009

Django WSGI handlers

WSGI handlers in the Django Python web application framework are used to provide the WSGI Python interface. This means that the Django framework can easily support middleware projects that manipulate the response sent back to the client. The code for the WSGIHandler class and the supporting WSGIRequest class can be found in the Python module. Here is a simple illustration of what the module provides.

Just as a brief overview of what this module contains, I'll simply describe each of the four classes shown above. First, the HttpRequest class provide the basic HTTP request attributes and methods. The BaseHandler class provides attributes and methods common to all handlers. The WSGIRequest class is a specialized HTTP request. The WSGIHandler class is a specialized handler.

The WSGIHandler class depends on the WSGIRequest class because it instantiates the request instances. This is done by setting the WSGIRequest class as the request_class attribute. There are two aspects of the WSGIHandler class that I find interesting.

First, this class isn't instantiated. There is no __init__() method defined and all attributes are class attributes. However, the __call__() method is defined so this class can actually be invoked is if we were creating an instance only we get a response object returned instead. At the beginning of the __call__() method, the WSGIHandler class acquires a threading lock. This lock is than released after the middleware has been loaded.

Second, each middleware method is than cumulatively invoked on the response so as to filter it. Very cool!