Routes functional conditions and WSGI Middleware

Sometimes, it amazes me whats possible fully utilizing WSGI middleware in an application stack. While it likely isn't something totally unique to the framework, the relative ease with which it can be done still sometimes gets me to grin.

Tonight, a Pylons user on the IRC channel (irc.freenode.net, #pylons) asked if it was possible to get a URL laid out so that /s/SOMETHING would map into their ’s' controller, with the second part passed in as a variable. That alone is pretty easy, however the additional requirement was that the controller action would change depending on the user's “type”.

There's two ways to deal with this, the first of which is the only possible way in many frameworks. Have every request to the URL map to a single function, and in that function load up the session and call the appropriate function to handle the request based on their user type. This way works fine in Pylons too, but thanks to Routes and WSGI middleware we have another option.

Routes has a lot of capabilities to it, there's been numerous additions to the Python implementation that the Rails version is not capable of. One of them is the ability to alter the resulting URL match dictionary in various conditional functions. To toggle the controller action used, we'll be using the ability to pass in a function to Routes conditions that can alter the resulting match.

This condition checking function has full access to the WSGI environ so if you wanted to restrict a specific controller/action combination to people referred from Slashdot, no problem! You can carefully fine-tune the conditions required for dispatch at the same place you define your URL resolution.

Since Pylons uses Beaker for session handling via WSGI middleware, the session object will already be available when our Pylons app gets called. Beaker loads the user session into environ['beaker.session']. Given this knowledge, we can write a conditional function for use with Routes like so:

def check_user(environ, result):
    session = environ['beaker.session']
    user_type = session.get('type')
    if not user_type:
        result['action'] = 'index'
    elif user_type == 'admin':
        result['action'] = 'view_action'
    else:
        result['action'] = 'not_logged_in'
    return True
map.connect('s/:domain', controller='s', conditions=dict(function=check_user))

Viola! Now Routes will run the function provided to see if it returns True before accepting that as a valid match. In the process, the action used will be set as desired. I've always thought a good sign something is well designed is when people can use it in ways you didn't originally anticipate. If that's the criteria, I think Routes succeeds and then some.

Disclaimer: Yes, I wrote Routes, and a good chunk of Beaker and Pylons, so I might be biased and tooting my own horn. :)