Beginning Python - From Novice to Professional

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CHAPTER 15 ■ PYTHON AND THE WEB 323 ■Caution This also means that everyone who has a user account on the machine can edit your file. You should be extremely cautious about this, especially if the file you are modifying is accessible through your Web server. If you are in doubt, ask your ISP or system administrator for advice. See also the following section, “CGI Security Risks.” CGI Security Risks Note that there are security issues involved in using CGI programs. If you allow your CGI script to write to files on your server, that ability may be used to destroy data unless you code your program carefully. Similarly, if you evaluate data supplied by a user as if it were Python code (for example, with exec or eval) or as a shell command (for example, with os.system or using the subprocess module), you risk performing arbitrary commands, which is a huge (as in humongous) risk. For a relatively comprehensive source of information about Web security, see the World Wide Web Consortium’s security FAQ (http://www.w3.org/Security/Faq). See also the security note on the subject in the Python Library Reference (http://python.org/doc/ lib/cgi-security.html). A Simple CGI Script The simplest possible CGI script looks something like Listing 15-4. Listing 15-4. A Simple CGI Script #!/usr/bin/env python print 'Content-type: text/plain' print # Prints an empty line, to end the headers print 'Hello, world!' If you save this in a file called simple1.cgi and open it through your Web server, you should see a Web page containing only the words “Hello, world!” in plain text. To be able to open this file through a Web server, you must put it where the Web server can access it. In a typical UNIX environment, putting it in a directory called public_html in your home directory would enable you to open it with the URL http://localhost/~username/simple1.cgi (substitute your user name for username). Ask your ISP or system administrator for details. As you can see, everything the program writes to standard output (for example, with print) ends up in the resulting Web page—at least almost. The fact is that the first things you print are HTTP headers—lines of information about the page. The only header I concern myself with here is Content-type. As you can see, the phrase “Content-type” is followed by a colon, a space, and the type name text/plain. This indicates that the page is plain text; to indicate HTML, this line should instead be print 'Content-type: text/html'
324 CHAPTER 15 ■ PYTHON AND THE WEB ■Note The names text/plain and text/html refer to MIME (Multipurpose Internet Mail Extensions) types. For a list of official MIME types, see http://www.iana.org/assignments/media-types. After all the headers have been printed, a single empty line is printed to signal that the document itself is about to begin. And, as you can see, in this case the document is simply the string "Hello, world!". Debugging with cgitb Sometimes a programming error makes your program terminate with a stacktrace due to an uncaught exception. When running the program through CGI, this will most likely result in an unhelpful error message from the Web server. In Python 2.2, a new module called cgitb (for CGI traceback) was added to the standard library. By importing it and calling its enable function, you can get a quite helpful Web page with information about what went wrong. Listing 15-5 gives an example of how you might use the cgitb module. Listing 15-5. A CGI Script That Invokes a Traceback (faulty.cgi) #!/usr/bin/env python import cgitb; cgitb.enable() print 'Content-type: text/html' print print 1/0 print 'Hello, world!' The result of accessing this script in a browser (through a Web server) is shown in Figure 15-1. Note that you might want to turn off the cgitb functionality after developing the program; the traceback page isn’t meant for the casual user of your program. (An alternative is to turn off the display and log the errors to files instead. See the Python Library Reference for more information.)
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Beginning Python From Novice to Pro
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256 CHAPTER 11 ■ FILES AND STUFF
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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