Dive Into Python
Python from novice to pro
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Dive Into PythonPython from novice to pro |
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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.
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Last-Modified/If-Modified-Since |
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Compression | |