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Mangling RSS feeds with Python


There are blogs on the web, that are written/configured in a way, that the RSS or Atom feed contains only a teaser (or no content at all), and one must open a link to get the real content – and thus load all the crap on the page, something RSS feeds were designed to avoid. Dittygirl has added one of those sites in her feed reader, and told me that it takes lots of resources on her netbook to load the whole page – not to mention the discomfort of leaving the feed reader.

I accepted the challenge, and decided to write a Python RSS gateway in less than 30 minutes. I chose plain WSGI, something I wanted to play with, and this project was a perfect match for its simplicity and lightweightness. Plain WSGI applications are Python modules with a callable named application, which the webserver will call every time, an HTTP request is made. The callable gets two parameters,

  • a dictionary of environment values (including the Path of the query, IP address of the browser, etc.), and
  • a callable, which can be used to signal the web server about the progress.

In this case, the script ignores the path, so only the second parameter is used.

def application(environ, start_response):
  rss = getfeed()
  response_headers = [('Content-Type', 'text/xml; charset=UTF-8'),
                      ('Content-Length', str(len(rss)))]
  start_response('200 OK', response_headers)
  return [rss]

Simple enough, the function emits a successful HTTP status, the necessary headers, and returns the content. The list (array) format is needed because a WSGI application can be a generator too (using a yield statement), which can be handy when rendering larger content, so the server expects an iterable result.

The real “business logic” is in the getfeed function, which first tries to load a cache, to avoid abusing the resources of the target server. I chose JSON as it's included in the standard Python libraries, and easy to debug.

  with open(CACHE, 'rb') as f:
    cached = json.load(f)
  etag = cached['etag']
  etag = ''

Next, I load the original feed, using the cached ETag value to encourage conditional HTTP GET. The urllib2.urlopen function can operate on a Request object, which takes a third parameter, that can be used to add HTTP headers. If the server responds with a HTTP 304 Not Modified, urlopen raises an HTTPError, and the script knows that the cache can be used.

  feedfp = urlopen(Request('http://HOSTNAME/feed/',
      None, {'If-None-Match': etag}))
except HTTPError as e:
  if e.code != 304:
  return cached['content'].encode('utf-8')

I used lxml to handle the contents, as it's a really convenient and fast library for XML/HTML parsing and manipulation. I compiled the XPath queries used for every item in the head of the module for performance reasons.

GUID = etree.XPath('guid/text()')
IFRAME = etree.XPath('iframe')
DESC = etree.XPath('description')

To avoid unnecessary copying, lxml's etree can parse the object returned by urlopen directly, and returns an object, which behaves like a DOM on steroids. The GUID XPath extracts the URL of the current feed item, and the HTML parser of lxml takes care of it. The actual contents of the post is helpfully put in a div with the class post-content, so I took advantage of lxml's HTML helper functions to get the div I needed.

While I was there, I also removed the first iframe from the post, which contains the Facebook tracker bug Like button. Finally, I cleared the class attribute of the div element, and serialized its contents to HTML to replace the useless description of the feed item.

feed = etree.parse(feedfp)
for entry in feed.xpath('/rss/channel/item'):
  ehtml = html.parse(GUID(entry)[0]).getroot()
  div = ehtml.find_class('post-content')[0]
  div.set('class', '')
  DESC(entry)[0].text = etree.CDATA(etree.tostring(div, method="html"))

There are two things left. First, the URL that points to the feed itself needs to be modified to produce a valid feed, and the result needs to be serialized into a string.

link = feed.xpath('/rss/channel/a:link',
  namespaces={'a': 'http://www.w3.org/2005/Atom'})[0]
link.set('href', 'http://URL_OF_FEED_GATEWAY/')
retval = etree.tostring(feed)

The second and final step is to save the ETag we got from the HTTP response and the transformed content to the cache in order to minimize the amount of resources (ab)used.

with open(CACHE, 'wb') as f:
  json.dump(dict(etag=feedfp.info()['ETag'], content=retval), f)
return retval

You might say, that it's not fully optimized, the design is monolithic, and so on – but it was done in less than 30 minutes, and it's been working perfectly ever since. It's a typical quick-and-dirty hack, and although it contains no technical breakthrough, I learned a few things, and I hope someone else might also do by reading it. Happy hacking!


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