Beginning Python - From Novice to Professional

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CHAPTER 15 ■ PYTHON AND THE WEB 321 Dynamic Web Pages with CGI This section deals with a basic Web programming technology: the Common Gateway Interface, or CGI. CGI is a standard mechanism by which a Web server can pass your queries (typically supplied through a Web form) to a dedicated program (for example, your Python program) and display the result as a Web page. It is a simple way of creating Web applications without writing your own special-purpose application server. For more information about CGI programming in Python, see the Web Programming Topic Guide on the Python Web site (http://python.org/ topics/web). The key tool in Python CGI programming is the cgi module. You can find a thorough description of it in the Python Library Reference (http://python.org/doc/lib/module-cgi.html). Another module that can be very useful during the development of CGI scripts is cgitb—more about that later. Before you can make your CGI scripts accessible (and runnable) through the Web, you need to put them where a Web server can access them, add a pound bang line, and set the proper file permissions. These three steps are explained in the following sections. Step 1. Preparing the Web Server I’m assuming that you have access to a Web server—in other words, that you can put stuff on the Web. Usually, that is a matter of putting your Web pages, images, and so on in a particular directory (in UNIX, typically called public_html). If you don’t know how to do this, you should ask your ISP or system administrator. Your CGI programs must also be put in a directory where they can be accessed via the Web. In addition, they must somehow be identified as CGI scripts, so the Web server doesn’t just serve the plain source code as a Web page. There are two typical ways of doing this: • Put the script in a subdirectory called cgi-bin. • Give your script the file name extension .cgi. Exactly how this works varies from server to server—again, check with your ISP or system administrator if you’re in doubt. (For example, if you’re using Apache, you may need to turn on the ExecCGI option for the directory in question.) Step 2. Adding the Pound Bang Line When you’ve put the script in the right place (and possibly given it a specific file name extension), you must add a pound bang line to the beginning of the script. I mentioned this in Chapter 1 as a way of executing your scripts without having to explicitly execute the Python interpreter. Usually, this is just convenient, but for CGI scripts it’s crucial; without it, the Web server won’t know how to execute your script. (For all it knows, the script could be written in some other programming language such as Perl or Ruby.) In general, simply adding the following line to the beginning of your script will do: #!/usr/bin/env python Note that it has to be the very first line. (No empty lines before it.) If that doesn’t work, you have to find out exactly where the Python executable is and use the full path in the pound bang line, as in the following:
322 CHAPTER 15 ■ PYTHON AND THE WEB #!/usr/bin/python If this doesn’t work, it may be that there’s something wrong that you cannot see, namely that the line ends in \r\n instead of simply \n, and your Web server gets confused. Make sure you’re saving the file as a plain UNIX-style text file. In Windows, you would use the full path to your Python binary, such as #!C:\Python22\python.exe Step 3. Setting the File Permissions The last thing you have to do (at least if your Web server is running on a UNIX or Linux machine) is to set the proper file permissions. You must make sure that everyone is allowed to read and execute your script file (otherwise the Web server wouldn’t be able to run it), but also make sure that only you are allowed to write to it (so nobody can change your script). ■Tip Sometimes, if you edit a script in Windows and it’s stored on a UNIX disk server (you may be accessing it through Samba or FTP, for example), the file permissions may be fouled up after you’ve made a change to your script. So if your script won’t run, make sure that the permissions are still correct. The UNIX command for changing file permissions (or file mode) is chmod. Simply run the following command (if your script is called somescript.cgi): chmod 755 somescript.cgi After having performed all these preparations, you should be able to open the script as if it were a Web page and have it executed. ■Note You shouldn’t open the script in your browser as a local file—you must open it with a full URL so that you actually fetch it via the Web (through your Web server). Your CGI script won’t normally be allowed to modify any files on your computer. If you want to allow it to change a file, you have to explicitly give it permission to do so. You have two options. If you have root (system administrator) privileges, you may create a specific user account for your script and change ownership of the files that need to be modified. If you don’t have root access, you can set the file permissions for the file so all users on the system (including that used by the Web server to run your CGI scripts) are allowed to write to the file. You can set the file permissions with this command: chmod 666 editable_file.txt
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Beginning Python From Novice to Pro
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Index ■Symbols - operator 558 !=
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