A Django Middleware Redirection class that provides a "last chance" to grab a developing 404 response and redirect it to a new URL.

redirectmiddleware.py

"""
This piece of middleware grabs onto the tail end of process_response hook
and allows for redirects to be applied using regex matches. 

Use Case: A legacy website with many old urls will have accumulated SEO mojo based
on external links. When the site is converted to a new fromat (say, drops the cruft like '.html')
or goes from abbreviated forms /art/id/t/234234 to /article/2014/january/technology/get-a-better-job
the old urls are at risk of breakage and could lose SEO mojo unless preserved

Best preservation comes from redirecting old content to new, which is easily accomplished
using 'django.contrib.redirects.middleware.RedirectFallbackMiddleware'. This class catches
the stuff that falls through the cracks; it gives the developer a last chance effort to 
catch and redirect. Say that "t" stood for technology in this url: /art/id/t/234234 
it might be a good idea to redirect unmatched /art/id/t/foobard to /article/2014/technology 
where the latter page lists an index of all articles by title

While you can perform a regex redirect in your http server, in urls.py, and in the django middleware,
that would miss the opportunity to preserve specific redirects that should live in the django Redirect table 

nginx --> urls --> 'django.contrib.redirects.middleware.RedirectFallbackMiddleware' -> LastChanceRedirect

*** NOTE WELL *** order is important this should be the last load in the middleware stack in settings.py
if you are using 'django.contrib.redirects.middleware.RedirectFallbackMiddleware' it must come after that!

MIDDLEWARE_CLASSES = (
    'django.middleware.common.CommonMiddleware',
    ...
    # put the redirectfallbackmiddleware end of list; order matters
    'django.contrib.redirects.middleware.RedirectFallbackMiddleware', 
    '<yourmoduledir>.redirectmiddleware.LastChanceRedirect',
)

https://docs.djangoproject.com/en/1.6/topics/http/middleware/
"""

from django import http
import re

redir_checks = {
	# 'regexpattern_for_old_path' : 'new/path/redirect' 
	'^/test/([\w\.]{2,20})$': '/newurl',
}

class LastChanceRedirect(object):

	# Defined as class-level attributes to be subclassing-friendly (from contrib.middlware.py)
    response_gone_class = http.HttpResponseGone
    response_redirect_class = http.HttpResponsePermanentRedirect

    def process_response(self, request, response):
        # only check 404 responses, all others should pass unmolested
        if response.status_code != 404:
            return response

        sought_url = request.get_full_path()
        matched = False

        try:
        	for key, value in redir_checks.items():
        		if re.match(key,sought_url,re.I):	
        			matched = True
        			new_path = value 
        except:
        	pass

        if matched:
			if new_path == '':
				return self.response_gone_class()
			else:
				return self.response_redirect_class(new_path)
				
        # Return the 'normal' response if a redirection was not found
        return response