Thursday, May 17, 2007

Python CGI Howto

I'm trying to use this blog as a place to document my new experiences with Python: some of them are very basic (like this post) and some are somewhat more advanced (for example the internal workings of Crunchy). So please don't be surprised at my "noobie" take on some things...

Anyway, the other day I was playing around with HacketyHack (just to know my enemy, as it were). Overall, I was quite impressed by it - although it does have slightly different aims to Crunchy. One of the best "features" of HacketyHack is its overall polish, and this was largely due to its excellent website. So I decided to make use of the Sourceforge webspace that Crunchy has had for a while (it used to be our main site) and at the same time learn how CGI works.

In case you didn't know, CGI stands for Common Gateway Interface and allows web pages to be generated on the fly by arbitrary programs. CGI works by literally sending printed output from the program down a pipe to the user's browser. Python, of course, has libraries to make CGI scripting easy. Here is a very simple python CGI script:

import cgi


I know that normally you wouldn't worry about that first line, but for CGI scripts it is absolutely essential! (without it you get an Internal Server Error). The second line imports Python's special CGI helper library and which includes a useful test function that we call in the final line. To make it work you have to place this script in your cgi-bin/ directory and give it global read and execute permissions using the *nix command:

$ chmod +rx filename

Once you've done this you can visit the page in your browser (it will probably be at a path something like /cgi-bin/, depending on how you named your script). You should see a detailed description of the environment in which you're script is running, but it won't look very pretty.

Writing a proper hello world CGI script is very simple, it looks like this:


print "Content-type: text/html"

print "<html><body>Hello World</body></html>"

Some more explanation: we don't actually need the cgi helper library here, so we don't load it. The first line of python code prints out a MIME-type that tells the browser what is coming next, then we print a blank line to indicate that we have finished generating headers, finally we print out the HTML code that we want to send.

Of course this is just a very short introduction to CGI scripting with Python, and if you want to create a larger dynamic website in Python you're probably better off using one of the Python web frameworks (like Twisted or Pylons). One day I might write a post about them...

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