Dive Into Python

11.2. How not to fetch data over HTTP

Let's say you want to download a resource over HTTP, such as a syndicated Atom feed. But you don't just want to download it once; you want to download it over and over again, every hour, to get the latest news from the site that's offering the news feed. Let's do it the quick-and-dirty way first, and then see how you can do better.

Example 11.2. Downloading a feed the quick-and-dirty way

>>> import urllib
>>> data = urllib.urlopen('http://diveintomark.org/xml/atom.xml').read()    
>>> print data
<?xml version="1.0" encoding="iso-8859-1"?>
<feed version="0.3"
  xmlns="http://purl.org/atom/ns#"
  xmlns:dc="http://purl.org/dc/elements/1.1/"
  xml:lang="en">
  <title mode="escaped">dive into mark</title>
  <link rel="alternate" type="text/html" href="http://diveintomark.org/"/>
  <-- rest of feed omitted for brevity -->

Downloading anything over HTTP is incredibly easy in Python; in fact, it's a one-liner. The urllib module has a handy urlopen function that takes the address of the page you want, and returns a file-like object that you can just read() from to get the full contents of the page. It just can't get much easier.

So what's wrong with this? Well, for a quick one-off during testing or development, there's nothing wrong with it. I do it all the time. I wanted the contents of the feed, and I got the contents of the feed. The same technique works for any web page. But once you start thinking in terms of a web service that you want to access on a regular basis -- and remember, you said you were planning on retrieving this syndicated feed once an hour -- then you're being inefficient, and you're being rude.

Let's talk about some of the basic features of HTTP.