Python CGI Programming

Until now we did a lot of stuffs with programming that are not related to web or network. Now it’s time for CGI. As the name suggests, CGI means “Common” gateway interface for everything. CGI is one of the important parts of HTTP (Hyper-Text Transfer Protocol).

It is a set of standards that defines a standard way of passing information or web-user request to an application program & to get data back to forward it to users. This is the exchange of information between web-server & a custom script. When the users requested the web-page the server sends the requested web-page. The web server usually pass the information to all application programs that processes data and sends back an acknowledged message; and this technique of passing data back-&-forth between server & application is the Common Gateway Interface. The current version of CGI is CGI/1.1 & CGI/1.2 is under process.

Browsing

What happens when a user clicks a hyperlink to browse a particular web-page or URL (Uniform Resource Locator).

The steps are:

Browser contacts the HTTP web server for demanding the URL
Parsing the URL
Look for the filename
If it finds that file, a request is send back
Web browser takes a response from web server
As the server response, it either shows the received file or an error message.

It may become possible to set-up an HTTP server because when a certain directory is requested that file is not sent back; instead it is executed as a program and that program’s output is displayed back to your browser.

Configuring CGI

The steps are:

Find out which user is running the Web-server
Check for server configuration to see if you can run the scripts in a particular directory
Check for file’s permission
Make a clear assurance that scripts you made are readable and executable by the web-server user
Make sure the Python-Script’s first line refers to web-server that the interpreter can run

The architecture of CHI is shown below:

Python CGI Program Structure

The output of Python CGI script must consist of two sections separated by a blank line. The first part contains the number of headers that describes the client what kind of data is following.

Python code header section looks something like this:

Example:

#!/usr/bin/python
print "Content-Type : text/html"

# then comes the rest hyper-text documents
print "<html>"
 print "<head>"
  print "<title>My First CGI-Program </title>"
 print "<head>"
 print "<body>"
  print "<h3>This is HTML's Body section </h3>"
 print "</body>"
print '</html>'

Save this file as CGI.py. When you will open that saved file, the output becomes:

Output:

This is HTML's Body section

This is a simple Python script that writes its output on STDOUT file i.e. on screen.

Use Of CGI Module

We programmers write CGI scripts in Python, they can add these lines:

import cgitb
cgitb.enable()

The above code triggers special exception handler that will display detailed report in the web-browser in case of occurrence of any error.

HTTP Header

Few are the important lists of HTTP header frequently used in CGI programs. These are:

HTTP header	Value
Content-type	text/html
Expires	Date
Location	URL
Set-Cookie	String
Content-length	N

CGI Environment Variables

Environment Variables	Description
CONTENT_TYPE	describes the data-type of the content
HTTP_COOKIE	returns the visitor’s cookie, if one is set
CONTENT_LENGTH	It is available for POST request to define the length of query information
HTTP_USER_AGENT	defines the browser type of the visitor
PATH_INFO	defines the path of CGI script
REMOTE_HOST	defines the host-name of the visitor
REMOTE_ADDR	defines the IP Address of the visitor
REQUEST_METHOD	used to make request & the most common methods are – GET and POST

Coding Theory

Search This Blog