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All of the HTTP responses we have seen so far specify the HTTP/1.1 protocol version, the return code 200, and the message OK. This indicates that each page was fetched successfully. But there are many more possible response codes. The full list is, of course, in RFC 2616, but here are the most basic responses (and we will discover a few others as this chapter progresses): 200 OK: The request has succeeded. 301 Moved Permanently: The resource that used to live at this URL has been assigned a new URL, which is specified in the Location: header of the HTTP response. And any bookmarks or other local copies of the link can be safely rewritten to the new URL.
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CHAPTER 9 HTTP
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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.
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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: ...
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CHAPTER 9 HTTP
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Server: Apache Content-Length: 285 Connection: close Content-Type: text/html; charset=iso-8859 1 As you can see, this particular web site does include a human-readable document with a 404 error; the response declares it to be an HTML page that is exactly 285 octets in length. (We will learn more about content length and types later in the chapter.) Like any HTTP response object, this exception can be queried for its status code; it can also be read like a file to see the returned page: >>> e.code 404 >>> e.msg 'Not Found' >>> e.readline() '<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">\n' If you try reading the rest of the file, then deep inside of the HTML you will see the actual error message that a web browser would display for the user: >>> e.read() '...The requested URL /better-living-through-http was not found on this server...' Redirections are very common on the World Wide Web. Conscientious web site programmers, when they undertake a major redesign, will leave 301 redirects sitting at all of their old-style URLs for the sake of bookmarks, external links, and web search results that still reference them. But the volume of redirects might be even greater for the many web sites that have a preferred host name that they want displayed for users, yet also allow users to type any of several different hostnames to bring the site up. The issue of whether a site name begins with www` looms very large in this area. Google, for example, likes those three letters to be included, so an attempt to open the Google home page with the hostname google.com will be met with a redirect to the preferred name: >>> info = opener.open('http://google.com/') -------------------------------------------------GET / HTTP/1.1 ... Host: google.com ... -------------------- Response -------------------HTTP/1.1 301 Moved Permanently Location: http://www.google.com/ ... -------------------------------------------------GET / HTTP/1.1 ... Host: www.google.com ... -------------------- Response -------------------HTTP/1.1 200 OK ... You can see that urllib2 has followed the redirect for us, so that the response shows only the final 200 response code: >>> info.code 200
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