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CHAPTER 9 HTTP
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You may have noticed that the HTTP request we opened the chapter with advertised the fact that it was generated by a Python program: User-Agent: Python-urllib/2.6 This header is optional in the HTTP protocol, and many sites simply ignore or log it. It can be useful when sites want to know which browsers their visitors use most often, and it can sometimes be used to distinguish search engine spiders (bots) from normal users browsing a site. For example, here are a few of the user agents that have hit my own web site in the past few minutes: Mozilla/5.0 (compatible; bingbot/2.0; +http://www.bing.com/bingbot.htm) Mozilla/5.0 (compatible; YandexBot/3.0; +http://yandex.com/bots) Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727) Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/534.3 (KHTML, like Gecko) Chrome/6.0.472.62 Safari/534.3 You will note that, the urllib2 user agent string notwithstanding, most clients choose to identify themselves as some form of the original Netscape browser, whose internal code name was Mozilla. But then, in parentheses, these same browsers secretly admit that they are really some other kind of browser. Many web sites are sensitive to the kinds of browsers that view them, most often because their designers were too lazy to make the sites work with anything other than Internet Explorer. If you need to access such sites with urllib2, you can simply instruct it to lie about its identity, and the receiving web site will not know the difference: >>> url = 'https://wca.eclaim.com/' >>> urllib2.urlopen(url).read() '<HTML>...The following are...required...Microsoft Internet Explorer...' >>> agent = 'Mozilla/5.0 (Windows; U; MSIE 7.0; Windows NT 6.0; en-US)' >>> request = urllib2.Request(url) >>> request.add_header('User-Agent', agent) >>> urllib2.urlopen(request).read() '\r\n<HTML>\r\n<HEAD>\r\n\t<TITLE>Eclaim.com - Log In</TITLE>...' There are databases of possible user agent strings online at several sites that you can reference both when analyzing agent strings that your own servers have received, as well as when concocting strings for your own HTTP requests: http://www.zytrax.com/tech/web/browser_ids.htm http://www.useragentstring.com/pages/useragentstring.php Besides using the agent string to enforce compatibility requirements usually in an effort to reduce development and support costs some web sites have started using the string to detect mobile browsers and redirect the user to a miniaturized mobile version of the site for better viewing on phones and iPods. A Python project named mobile.sniffer that attempts to support this technique can be found on the Package Index.
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CHAPTER 9 HTTP
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It is always possible to simply make an HTTP request and let the server return a document with whatever Content-Type: is appropriate for the information we have requested. Some of the usual content types encountered by a browser include the following: text/html text/plain text/css image/gif image/jpeg image/x-png application/javascript application/pdf application/zip If the web service is returning a generic data stream of bytes that it cannot describe more specifically, it can always fall back to the content type: application/octet-stream But some clients do support all content types. Such clients like to encourage servers to send compatible content when several versions of a resource are available. This selection can occur along several axes: older browsers might not know about new, up-and-coming image formats; some browsers can only read certain encodings; and, of course, each user has particular languages that she can read and prefers web sites to deliver content in her native tongue, if possible. Consult RFC 2616 if you find that your Python web client is sophisticated enough that you need to wade into content negotiation. The four headers that will interest you include the following: Accept Accept-Charset Accept-Language Accept-Encoding Each of these headers supports a comma-separated list of items, where each item can be given a weight between one and zero (larger weights indicate more preferred items) by adding a suffix that consists of a semi-colon and q= string to the item. The result will look something like this (using, for illustration, the Accept: header that my Google Chrome browser seems to be currently using): Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain; q=0.8,image/png,*/*;q=0.5 This indicates that Chrome prefers XML and XHTML, but will accept HTML or even plain text if those are the only document formats available; that Chrome prefers PNG images when it can get them; and that it has no preference between all of the other content types in existence. The HTTP standard also describes the possibility of a client receiving a 300 Multiple Choices response and getting to choose its own content type; however, this does not seem to be a widelyimplemented mechanism, and I refer you to the RFC should you ever need to use it.
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