Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 9 ■ HTTP
• 303 See Other: The original URL should continue to be used for this request, but
on this occasion the response can be found by retrieving a different URL—the one
in the response’s Location: header. If the operation was a POST or PUT (which we
will learn about later in this chapter), then a 303 means that the operation has
succeeded, and that the results can be viewed by doing a GET at the new location.
• 304 Not Modified: The response would normally be a 200 OK, but the HTTP request
headers indicate that the client already possesses an up-to-date copy of the
resource, so its body need not be transmitted again, and this response will contain
only headers. See the section on caching later in this chapter.
• 307 Temporary Redirect: This is like a 303, except in the case of a POST or PUT,
where a 307 means that the action has not succeeded but needs to be retried with
another POST or PUT at the URL specified in the response Location: header.
• 404 Not Found: The URL does not name a valid resource.
• 500 Internal Server Error: The web site is broken. Programmer errors,
configuration problems, and unavailable resources can all cause web servers to
generate this code.
• 503 Service Unavailable: Among the several other 500-range error messages, this
may be the most common. It indicates that the HTTP request cannot be fulfilled
because of some temporary and transient service failure. This is the code included
when Twitter displays its famous Fail Whale, for example.
Each HTTP library makes its own choices about how to handle the various status codes. If its full
stack of handlers is left in place, urllib2 will automatically follow redirections. Return codes that cannot
be handled, or that indicate any kind of error, are raised as Python exceptions:
>>> nonexistent_url = 'http://example.com/better-living-through-http'
>>> response = opener.open(nonexistent_url)
Traceback (most recent call last):
...
HTTPError: HTTP Error 404: Not Found
But these exception objects are special: they also contain all of the usual fields and capabilities of
HTTP response information objects. Remember that many web servers include a useful human-readable
document when they return an error status. Such a document might include specific information about
what has gone wrong. For example, many web frameworks—at least when in development mode—will
return exception tracebacks along with their 500 errors when the program trying to generate the web
page crashes.
By catching the exception, we can both see how the HTTP response looked on the wire (thanks
again to the special handler that we have installed in our opener object), and we can assign a name to the
exception to look at it more closely:
>>> try:
... response = opener.open(nonexistent_url)
... except urllib2.HTTPError, e:
... pass
--------------------------------------------------
GET /better-living-through-http HTTP/1.1
...
-------------------- Response --------------------
HTTP/1.1 404 Not Found
Date: ...
145