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

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CHAPTER 9 ■ HTTP Relative URLs Very often, the links used in web pages do not specify full URLs, but relative URLs that are missing several of the usual components. When one of these links needs to be resolved, the client needs to fill in the missing information with the corresponding fields from the URL used to fetch the page in the first place. Relative URLs are convenient for web page designers, not only because they are shorter and thus easier to type, but because if an entire sub-tree of a web site is moved somewhere else, then the links will keep working. The simplest relative links are the names of pages one level deeper than the base page: >>> urlparse.urljoin('http://www.python.org/psf/', 'grants') 'http://www.python.org/psf/grants' >>> urlparse.urljoin('http://www.python.org/psf/', 'mission') 'http://www.python.org/psf/mission' Note the crucial importance of the trailing slash in the URLs we just gave to the urljoin() function! Without the trailing slash, the call function will decide that the current directory (called officially the base URL) is / rather than /psf/; therefore, it will replace the psf component entirely: >>> urlparse.urljoin('http://www.python.org/psf', 'grants') 'http://www.python.org/grants' Like file system paths on the POSIX and Windows operating systems, . can be used for the current directory and .. is the name of the parent: >>> urlparse.urljoin('http://www.python.org/psf/', './mission') 'http://www.python.org/psf/mission' >>> urlparse.urljoin('http://www.python.org/psf/', '../news/') 'http://www.python.org/news/' >>> urlparse.urljoin('http://www.python.org/psf/', '/dev/') 'http://www.python.org/dev' And, as illustrated in the last example, a relative URL that starts with a slash is assumed to live at the top level of the same site as the original URL. Happily, the urljoin() function ignores the base URL entirely if the second argument also happens to be an absolute URL. This means that you can simply pass every URL on a given web page to the urljoin() function, and any relative links will be converted; at the same time, absolute links will be passed through untouched: # Absolute links are safe from change >>> urlparse.urljoin('http://www.python.org/psf/', 'http://yelp.com/') 'http://yelp.com/' As we will see in the next chapter, converting relative to absolute URLs is important whenever we are packaging content that lives under one URL so that it can be displayed at a different URL. Instrumenting urllib2 We now turn to the HTTP protocol itself. Although its on-the-wire appearance is usually an internal detail handled by web browsers and libraries like urllib2, we are going to adjust its behavior so that we can see the protocol printed to the screen. Take a look at Listing 9–1. 141
CHAPTER 9 ■ HTTP Listing 9–1. An HTTP Request and Response that Prints All Headers #!/usr/bin/env python # Foundations of Python Network Programming - Chapter 9 - verbose_handler.py # HTTP request handler for urllib2 that prints requests and responses. import StringIO, httplib, urllib2 class VerboseHTTPResponse(httplib.HTTPResponse): » def _read_status(self): » » s = self.fp.read() » » print '-' * 20, 'Response', '-' * 20 » » print s.split('\r\n\r\n')[0] » » self.fp = StringIO.StringIO(s) » » return httplib.HTTPResponse._read_status(self) class VerboseHTTPConnection(httplib.HTTPConnection): » response_class = VerboseHTTPResponse » def send(self, s): » » print '-' * 50 » » print s.strip() » » httplib.HTTPConnection.send(self, s) class VerboseHTTPHandler(urllib2.HTTPHandler): » def http_open(self, req): » » return self.do_open(VerboseHTTPConnection, req) To allow for customization, the urllib2 library lets you bypass its vanilla urlopen() function and instead build an opener full of handler classes of your own devising—a fact that we will use repeatedly as this chapter progresses. Listing 9–1 provides exactly such a handler class by performing a slight customization on the normal HTTP handler. This customization prints out both the outgoing request and the incoming response instead of keeping them both hidden. For many of the following examples, we will use an opener object that we build right here, using the handler from Listing 9–1: >>> from verbose_http import VerboseHTTPHandler >>> import urllib, urllib2 >>> opener = urllib2.build_opener(VerboseHTTPHandler) You can try using this opener against the URL of the RFC that we mentioned at the beginning of this chapter: opener.open('http://www.ietf.org/rfc/rfc2616.txt') The result will be a printout of the same HTTP request and response that we used as our example at the start of the chapter. We can now use this opener to examine every part of the HTTP protocol in more detail. The GET Method When the earliest version of HTTP was first invented, it had a single power: to issue a method called GET that named and returned a hypertext document from a remote server. That method is still the backbone of the protocol today. 142
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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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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 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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CHAPTER 11 ■ WEB APPLICATIONS the
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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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