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

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CHAPTER 9 ■ HTTP POST And APIs Almost none of the caveats given in the last two sections apply when an HTTP POST is designed for consumption by a program other than a web browser. This is because all of the issues that hinge upon user interaction, browser history, and the “reload” and “back” buttons will simply not apply. To begin with, a POST designed for use by a program need not use the awkward x-www-formurlencoded data format for its input parameters. Instead, it can specify any combination of content type and input data that its programmer is prepared for it to handle. Data formats like XML, JSON, and BSON are often used. Some services even allow entire documents or images to be posted raw, so long as the request Content-Type: header is set to correctly indicate their type. The next common difference is that API calls made through POST rarely result in redirection; sending a program to another URL to receive the result of such calls (and thus requiring the client to make a second round-trip to the server to perform that download) only makes sense if the whole point of the service is to map requests into references to other URLs. Finally, the range of payload content types returned from API calls is much broader than the kinds of data that can usefully be returned to browsers. Instead of supporting only things like web pages, style sheets, and images, the programs that consume web APIs often welcome rich formatted data like that supported by formats like XML and JSON. Often, a service will choose the same data format for both its POST request and return values, and thus require client programs to use only one data library for coercion, rather than two. Note that many API services are designed for use with a JavaScript program running inside of a web page delivered through a normal GET call. Despite the fact that the JavaScript is running in a browser, such services will act like APIs rather than user form posts: they typically do not redirect, but instead send and receive data payloads (typically) rather than browsable web pages. REST And More HTTP Methods We have just introduced the topic of web-based APIs, which fetch documents and data using GET and POST to specific URLs. Therefore, we should immediately note that many modern web services try to integrate their APIs more tightly with HTTP by going beyond the two most common HTTP methods by implementing additional methods like PUT and DELETE. In general, a web API that is implemented entirely with POST commands remains opaque to proxies, caches, and any other tools that support the HTTP protocol. All they know is that a series of unrepeatable special commands are passing between the client and the server. But they cannot detect whether resources are being queried, created, destroyed, or manipulated. A design pattern named “Representational State Transfer” has therefore been taking hold in many developer communities. This design pattern is based on Roy Fielding’s celebrated 2000 doctoral dissertation that first fully defined the concept. It specifies that the nouns of an API should live at their own URLs. For example, PUT, GET, POST, and DELETE should be used, respectively, to create, fetch, modify, and remove the documents living at these URLs. By coupling this basic recommendation with further guidelines, the REST methodology guides the creation of web services that make more complete use of the HTTP protocol (instead of treating it as a dumb transport mechanism). Such web services also offer quite clean semantics, and can be accelerated by the same caching proxies that are often used to speed the delivery of normal web pages. There are now entire books dedicated to RESTful web services, which I recommend you peruse if you are going to be building programmer web interfaces in the future! Note that HTTP supports arbitrary method names, even though the standard defines specific semantics for GET and POST and all of the rest. Tradition would dictate using the well-known methods defined in the standard unless you are using a specific framework or methodology that recognizes and has defined other methods. 151
CHAPTER 9 ■ HTTP Identifying User Agents and Web Servers 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() '...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\r\n\r\n\tEclaim.com - Log In...' 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. 152
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Foundations of Python Network Progr
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To the Python community for creatin
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Contents ■Contents at a Glance ..
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■ CONTENTS Asking getaddrinfo() W
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■ CONTENTS Using Message Queues f
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■ CONTENTS Parsing Dates ........
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■ CONTENTS Telnet ...............
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About the Authors ■ Brandon Craig
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Acknowledgements This book owes its
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■ INTRODUCTION If you do know som
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C H A P T E R 1 ■ ■ ■ Introdu
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CHAPTER 1 ■ INTRODUCTION TO CLIEN
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C H A P T E R 2 ■ ■ ■ UDP The
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CHAPTER 2 ■ UDP server with SSH.
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CHAPTER 2 ■ UDP them anywhere in
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CHAPTER 2 ■ UDP command-line argu
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CHAPTER 2 ■ UDP » » » » raise
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CHAPTER 2 ■ UDP world itself give
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CHAPTER 2 ■ UDP socket that is no
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CHAPTER 2 ■ UDP So binding to an
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CHAPTER 2 ■ UDP s.connect((hostna
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CHAPTER 2 ■ UDP else: » print >>
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C H A P T E R 3 ■ ■ ■ TCP The
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CHAPTER 3 ■ TCP situation), and t
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CHAPTER 3 ■ TCP » reply = recv_a
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CHAPTER 3 ■ TCP guess when the in
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CHAPTER 3 ■ TCP the system has no
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CHAPTER 3 ■ TCP » » » print '\
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CHAPTER 3 ■ TCP $ python tcp_dead
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CHAPTER 3 ■ TCP Using TCP Streams
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CHAPTER 4 ■ SOCKET NAMES AND DNS
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CHAPTER 5 ■ NETWORK DATA AND NETW
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C H A P T E R 6 ■ ■ ■ TLS and
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CHAPTER 6 ■ TLS AND SSL systems a
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CHAPTER 6 ■ TLS AND SSL • He wi
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CHAPTER 6 ■ TLS AND SSL discussio
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CHAPTER 6 ■ TLS AND SSL • The s
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CHAPTER 6 ■ TLS AND SSL The Links
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C H A P T E R 7 ■ ■ ■ Server
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Page 144 and 145: C H A P T E R 8 ■ ■ ■ Caches,
Page 146 and 147: CHAPTER 8 ■ CACHES, MESSAGE QUEUE
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Page 162 and 163: CHAPTER 9 ■ HTTP From now on, I a
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Page 184 and 185: CHAPTER 10 ■ SCREEN SCRAPING Figu
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Page 216 and 217: C H A P T E R 12 ■ ■ ■ E-mail
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CHAPTER 12 ■ E-MAIL COMPOSITION A
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C H A P T E R 13 ■ ■ ■ SMTP A
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CHAPTER 13 ■ SMTP anyway. Outgoin
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CHAPTER 13 ■ SMTP How SMTP Is Use
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CHAPTER 13 ■ SMTP This mechanism
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CHAPTER 13 ■ SMTP s = smtplib.SMT
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CHAPTER 13 ■ SMTP ETRN STARTTLS X
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CHAPTER 13 ■ SMTP » s = smtplib.
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CHAPTER 13 ■ SMTP exchange mail o
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CHAPTER 13 ■ SMTP username = sys.
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C H A P T E R 14 ■ ■ ■ POP PO
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CHAPTER 14 ■ POP ■ Caution! Whi
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CHAPTER 14 ■ POP finally: » p.qu
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CHAPTER 14 ■ POP Subject: Backup
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CHAPTER 15 ■ IMAP THE IMAP PROTOC
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CHAPTER 15 ■ IMAP '(\\HasNoChildr
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CHAPTER 15 ■ IMAP Examining Folde
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CHAPTER 15 ■ IMAP Listing 15-5. D
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CHAPTER 15 ■ IMAP key that IMAP h
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CHAPTER 15 ■ IMAP » » print def
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CHAPTER 15 ■ IMAP » From: Brando
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CHAPTER 15 ■ IMAP • \Flagged: T
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CHAPTER 15 ■ IMAP An IMAP message
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CHAPTER 15 ■ IMAP display or summ
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CHAPTER 16 ■ TELNET AND SSH cloud
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CHAPTER 16 ■ TELNET AND SSH Unix
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CHAPTER 16 ■ TELNET AND SSH Do yo
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CHAPTER 16 ■ TELNET AND SSH As we
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CHAPTER 16 ■ TELNET AND SSH tabif
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CHAPTER 16 ■ TELNET AND SSH repla
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CHAPTER 16 ■ TELNET AND SSH Listi
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CHAPTER 16 ■ TELNET AND SSH def p
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CHAPTER 16 ■ TELNET AND SSH We wi
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CHAPTER 16 ■ TELNET AND SSH • p
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CHAPTER 16 ■ TELNET AND SSH You w
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CHAPTER 16 ■ TELNET AND SSH » »
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CHAPTER 16 ■ TELNET AND SSH Listi
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CHAPTER 16 ■ TELNET AND SSH Summa
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CHAPTER 17 ■ FTP The biggest prob
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CHAPTER 17 ■ FTP f.login() print
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CHAPTER 17 ■ FTP if os.path.exist
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CHAPTER 17 ■ FTP f = FTP(host) f.
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CHAPTER 17 ■ FTP Windows servers
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CHAPTER 17 ■ FTP » try: » » f.
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C H A P T E R 18 ■ ■ ■ RPC Re
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CHAPTER 18 ■ RPC sort of proxy ex
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CHAPTER 18 ■ RPC The SimpleXMLRPC
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CHAPTER 18 ■ RPC Traceback (most
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CHAPTER 18 ■ RPC 8.0 If this
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CHAPTER 18 ■ RPC Note that the po
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CHAPTER 18 ■ RPC up being, simply
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CHAPTER 18 ■ RPC such as Python i
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CHAPTER 18 ■ RPC • Google Proto
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■ INDEX mod_python, 194 Qpid, 131
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■ INDEX Common Gateway Interface.
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■ INDEX international characters
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■ INDEX front-end web servers, 17
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■ INDEX deleting folders, 260 del
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■ INDEX mechanize, 138, 163 Memca
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■ INDEX pausing terminal output,
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■ INDEX resources. See also RFCs
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■ INDEX shutdown(), 48 shutting d
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■ INDEX terminals, 270-74 bufferi
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■ INDEX ■ V validating cached r
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