The User-Agent is simply a way for a client to tell a server who it is when it requests a web page, a syndicated feed, or any sort of web service over HTTP. When the client requests a resource, it should always announce who it is, as specifically as possible. This allows the server-side administrator to get in touch with the client-side developer if anything is going fantastically wrong.

By default, Python sends a generic User-Agent: Python-urllib/1.15. In the next section, you'll see how to change this to something more specific.

Sometimes resources move around. Web sites get reorganized, pages move to new addresses. Even web services can reorganize. A syndicated feed at http://example.com/index.xml might be moved to http://example.com/xml/atom.xml. Or an entire domain might move, as an organization expands and reorganizes; for instance, http://www.example.com/index.xml might be redirected to http://server-farm-1.example.com/index.xml.

Every time you request any kind of resource from an HTTP server, the server includes a status code in its response. Status code 200 means “everything's normal, here's the page you asked for”. Status code 404 means “page not found”. (You've probably seen 404 errors while browsing the web.)

HTTP has two different ways of signifying that a resource has moved. Status code 302 is a temporary redirect; it means “oops, that got moved over here temporarily” (and then gives the temporary address in a Location: header). Status code 301 is a permanent redirect; it means “oops, that got moved permanently” (and then gives the new address in a Location: header). If you get a 302 status code and a new address, the HTTP specification says you should use the new address to get what you asked for, but the next time you want to access the same resource, you should retry the old address. But if you get a 301 status code and a new address, you're supposed to use the new address from then on.

urllib.urlopen will automatically “follow” redirects when it receives the appropriate status code from the HTTP server, but unfortunately, it doesn't tell you when it does so. You'll end up getting data you asked for, but you'll never know that the underlying library “helpfully” followed a redirect for you. So you'll continue pounding away at the old address, and each time you'll get redirected to the new address. That's two round trips instead of one: not very efficient! Later in this chapter, you'll see how to work around this so you can deal with permanent redirects properly and efficiently.

Some data changes all the time. The home page of CNN.com is constantly updating every few minutes. On the other hand, the home page of Google.com only changes once every few weeks (when they put up a special holiday logo, or advertise a new service). Web services are no different; usually the server knows when the data you requested last changed, and HTTP provides a way for the server to include this last-modified date along with the data you requested.

If you ask for the same data a second time (or third, or fourth), you can tell the server the last-modified date that you got last time: you send an If-Modified-Since header with your request, with the date you got back from the server last time. If the data hasn't changed since then, the server sends back a special HTTP status code 304, which means “this data hasn't changed since the last time you asked for it”. Why is this an improvement? Because when the server sends a 304, it doesn't re-send the data. All you get is the status code. So you don't need to download the same data over and over again if it hasn't changed; the server assumes you have the data cached locally.

All modern web browsers support last-modified date checking. If you've ever visited a page, re-visited the same page a day later and found that it hadn't changed, and wondered why it loaded so quickly the second time -- this could be why. Your web browser cached the contents of the page locally the first time, and when you visited the second time, your browser automatically sent the last-modified date it got from the server the first time. The server simply says 304: Not Modified, so your browser knows to load the page from its cache. Web services can be this smart too.

Python's URL library has no built-in support for last-modified date checking, but since you can add arbitrary headers to each request and read arbitrary headers in each response, you can add support for it yourself.

ETags are an alternate way to accomplish the same thing as the last-modified date checking: don't re-download data that hasn't changed. The way it works is, the server sends some sort of hash of the data (in an ETag header) along with the data you requested. Exactly how this hash is determined is entirely up to the server. The second time you request the same data, you include the ETag hash in an If-None-Match: header, and if the data hasn't changed, the server will send you back a 304 status code. As with the last-modified date checking, the server just sends the 304; it doesn't send you the same data a second time. By including the ETag hash in your second request, you're telling the server that there's no need to re-send the same data if it still matches this hash, since you still have the data from the last time.

Python's URL library has no built-in support for ETags, but you'll see how to add it later in this chapter.

The last important HTTP feature is gzip compression. When you talk about HTTP web services, you're almost always talking about moving XML back and forth over the wire. XML is text, and quite verbose text at that, and text generally compresses well. When you request a resource over HTTP, you can ask the server that, if it has any new data to send you, to please send it in compressed format. You include the Accept-encoding: gzip header in your request, and if the server supports compression, it will send you back gzip-compressed data and mark it with a Content-encoding: gzip header.

Python's URL library has no built-in support for gzip compression per se, but you can add arbitrary headers to the request. And Python comes with a separate gzip module, which has functions you can use to decompress the data yourself.

Note that our little one-line script to download a syndicated feed did not support any of these HTTP features. Let's see how you can improve it.