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

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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
CHAPTER 9 ■ HTTP 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() '\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 200 146
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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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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 158 and 159: CHAPTER 9 ■ HTTP Here, the URL sp
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Page 162 and 163: CHAPTER 9 ■ HTTP From now on, I a
Page 166 and 167: CHAPTER 9 ■ HTTP You cannot tell
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Page 174 and 175: CHAPTER 9 ■ HTTP HTTP Caching Man
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Page 182 and 183: C H A P T E R 10 ■ ■ ■ Screen
Page 184 and 185: CHAPTER 10 ■ SCREEN SCRAPING Figu
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CHAPTER 11 ■ WEB APPLICATIONS The
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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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