A Simple Python CGI Server Tutorial - Part 2 - CGI Input/Output

A Simple Python CGI Server Tutorial - Part 2 - CGI Input/Output

This article will deal with retrieving and using input to your CGI programs. There are two ways to get input from your CGI programs, encoded within the URL and submitted through forms. This tutorial will go over both methods.

We'll need to do a little setup to begin, if you've already read through the first tutorial you should be very familiar with this process.

First we need to create a directory to house our CGI server and scripts, for this tutorial we'll name it 'server'. Create a new file, with the following program, in the 'server' directory and name it 'server.py'.


#!/usr/bin/env python import BaseHTTPServer import CGIHTTPServer server = BaseHTTPServer.HTTPServer handler = CGIHTTPServer.CGIHTTPRequestHandler server_address = ("", 8000) handler.cgi_directories = [""] httpd = server(server_address, handler) httpd.serve_forever()


Give 'server.py' executable permissions ($ chmod +x server.py). The next step is to create a page that will give us a link with an encoded URL. To do this we'll create a simple HTML page with nothing but the encoded link in it. Create a new file named 'link.py' in the 'server' directory, with the following code and give it executable permissions ($ chmod +x link.py).


#!/usr/bin/env python print "Content-type: text/html" print print "<title>Test URL Encoding</title>" print '<p><a href="http://localhost:8000/test_urlencode.py?first=Jack&last=Trades">Link</a></p>'


Now we need to create the CGI script that will receive this data and output it into a new page. Create a new file named 'test_urlencode.py' in the 'server' directory and input the following code.


#!/usr/bin/env python import cgi form = cgi.FieldStorage() val1 = form.getvalue('first') val2 = form.getvalue('last') print "Content-type: text/html" print print "<title>Test URL Encoding</title>" print "<p>" print "Hello my name is " + val1 + " " + val2 print "</p>"


Remember to give executable permissions to all your CGI scripts ($ chmod +x test_urlencode.py). That's it! We now have a working CGI script. To test it type the following into the location bar of your browser.


http://localhost:8000/link.py

Click on the link and the 'test_urlencode.py' script will output a page that says "Hello my name is Jack Trades". You can play around with the output by changing the URL of the link to whatever you want. Below is a sample of URLs and their output, you can type/paste these directly into the location bar (no need to modify the 'link.py' script).

http://localhost:8000/test_urlencode.py?first=Jack&last=Trades
Hello my name is Jack Trades

http://localhost:8000/test_urlencode.py?first=Adam&last=Baum
Hello my name is Adam Baum

http://localhost:8000/test_urlencode.py?first=Anna&last=Grahm
Hello my name is Anna Grahm

Courtesy of Bart Simpson...
http://localhost:8000/test_urlencode.py?last=Tinkle&first=Ivana
Hello my name is Ivana Tinkle


OK, that's enough fun for now. Let's move on to retrieving input from form data. The first thing we must do is create a form to enter some data into. We'll do this by writing another CGI script to output the HTML form. Create another file in the 'server' directory named 'form.py' with the code below.

#!/usr/bin/env python print "Content-type: text/html" print print '<form method="post" action="test_form.py">' print '<textarea name="comments" cols="40" rows="5">' print 'Enter comments here...' print '</textarea>' print '<br>' print '<input type="submit" value="Submit">' print '</form>'


This is the form that will be presented to the user to allow them to enter their 'comments'. Remember to give this script executable permissions ($ chmod +x form.py). Next create another file in the 'server' directory named 'test_form.py', this is the CGI script that will accept the form data and create a new page that includes the data.


#!/usr/bin/env python import cgi form = cgi.FieldStorage() val1 = form.getvalue('comments') print 'Content-type: text/html' print print 'The form input is below...' print '<br>' print val1

Give this file executable permissions as well ($ chmod +x test_form.py) and then go back to your browser and enter the following line in the location bar.

http://localhost:8000/form.py

You should see a form with a single text area and a submit button. Type whatever you want to in the text area, I chose "la de da de, we like to party", and hit the submit button. You should be presented by a page that says...

The form input is below...


la de da de, we like to party

Congratulations you have successfully handled form data with a CGI script. You'll notice that Python's cgi module uses cgi.FieldStorage() to access both URL encoded input as well as form based input. I neglected to go over the scripts line-by-line because there are very few concepts introduced in each of them. If you have any questions, or would like me to clarify something, please leave a comment below.