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[siː dʒiː aɪ]

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The Common Gateway Interface (CGI) is a standard protocol for interfacing external application software with an information server, commonly a web server. This allows the server to pass requests from a client web browser to the external application. The web server can then return the output from the application to the web browser.

Wikipedia

Today, I’ll explain how to generate Web pages using Python - the easiest programming language ever. They won’t be just static pages, however; in the article there will be examples of CGI scripts processing data from HTML forms, uploading files to the server, some GET and POST requests and even a simple how-to showing how to set up a small CGI server. However, this article is about using CGI in Python, so I assume you have some knowledge of the Python syntax.

The simpliest of the simpliest

Let’s start with a simple static page. The first line of any CGI script has to point to the location of Python. It’ll usually be /usr/bin/python on most Unix-based servers. Next, we have to inform Python that the output of the script is actually HTML (and put an \n at the end of this line)- and the rest of the code is up to you. Remember that all CGI scripts need permissions at least equal to chmod 755.

#!/usr/bin/python
print "Content-type: text/html\\n"
print "OMG, it's actually working!"

You can see the script working.
Of course, this script doesn’t even include the most basic html tags such as <html> and <body>, so have a look at a second example:

#!/usr/bin/python
print "Content-type: text/html\\n"
print """<html>
<head>
<title>OMG</title>
</head>
<body>
<h2>It's actually working!</h2>
<img src="some_crappy_image_source">
</body>
</html>"""

Again, you can see how it works.

Mummy, my script isn’t working!

CGI scripts are difficult to write, as even the simpliest mistakes cause internal server errors (#500). Remember to put a new line sign (if your server is a Unix-based one, \n is the new line sign, and the DOS \r\n is not supported) at the end of the line that indicates the content type. Also, be sure to check if your server actually supports Python and CGI scripts. If the answer is yes, but the scripts still don’t work, ask the hosting support (e.g. on Dreamhost, without proper domain settins, you won’t be able to execute any CGI scripts). We’ll get to some other errors later.

Processing data

Of course, using CGI to generate static web pages is stupid. Now, I’ll show you how to write a data-processing script. First, the form:

#!/usr/bin/python
print "Content-type: text/html\\n"
print """<html>
<head><title>The form</title></head>
<body>
<form action="path-to-the-script.cgi" method="post">
<label for="login">Login: </label>
<input type="text" name="login">
<br>
<label for="pass">Password: </label>
<input type="text" name="pass">
<br><input type="submit" value="OK, let\\'s go">
</form>
</body>
</html>"""

Next, let’s write a script that will check whether the password equals to “pass123″ and display a nice message to the user. The function that we’ll use is cgi.FieldStorage(), which returns a dictionary of all GET and POST request variables.

#!/usr/bin/python
from cgi import FieldStorage as data
from sys import stderr, stdout
stderr = stdout
data = data()
print "Content-type: text/html\\n"
if data.has_key('login') and data.has_key('password'):
   if data['password'].value == 'pass123':
      print 'Hello, %s. You\\'ve entered a <b>correct password</b>!' % data['login'].value
   else:
      print 'Hello, %s. The password you\\'ve entered is incorrect.' % data['login'].value
else:
   print 'Fill all the form fields!'

Check it out.
Some explanation: we want errors (sys.stderr) to be displayed via the default output (sys.stdout), which is the data that goes to the browser and, as a result of that, to the user. I used has_hey() to check if all the fields were filled in, and then displayed a message. CGI handles GET request the same way, i.e. I could’ve sent the form data using the GET method and the script would still work properly. The FieldStorage function stores all requests from you to the server, so it even stores files you send via forms (see below).

Self-posting CGI scripts

Of course having two separate CGI scripts isn’t necessary and we can get the code into one file, which will be a self-posting script. Here’s the code of a simple guestbook saving the guest data to a file:

#!/usr/bin/python

from cgi import FieldStorage as data
from sys import stderr, stdout
stderr = stdout
data = data()
f = open('guestbook.txt', 'a+')

print "Content-type: text/html\\n"
print "<html><head><title>Guestbook</title></head>"
print "<body><h2>Guestbook</h2>"

if data.has_key('name') and data.has_key('comment'):
	f.write('<li><b>%s</b> said:<br>%s</li>' % (data['name'].value, data['comment'].value))
	print "Thanks for commenting on our site!<br>"
	print "<a href='path-to-the-guestbook.cgi'>See the guestbook</a>"
else:
	print '<ul>', f.read(), '</ul>'
	print "<br>Add your comment:<br><br>"
	print """<form action="path-to-the-guestbook.cgi" method="post">
Your name:<br>
<input type="text" name="name"><br>
Your comment:<br>
<textarea name="comment" rows="4" cols="50"></textarea><br>
<input type="submit" value="OK">
</form>"""

f.close()
print "</body></html>"

Now you can write some stupid things in my guestbook.
Needless to say, we need a guestbook.txt file with proper permitions for the guestbook to work.

File upload

When writing CGI scripts, you may want the users to be able to upload files. How to accomplish this? Again, using the FieldStorage function! Yeah, it’s extremely easy. In the following script, the information on all uploaded files is saved to a file list, which is read every time you enter the page without the actual file in FieldStorage. Notice the form enctype="multipart/form-data".

#!/usr/bin/python

from cgi import FieldStorage as data
from sys import stdout, stderr
stderr = stdout

data = data()

f = open('FILE-LIST', 'a+')

print 'Content-type: text/html\\n'

print '<html><head><title>Files</title></head><body><h2>Files</h2>'

if data.has_key('upload'):
        upload = data['upload']
        content = upload.file.read()
	size = len(content)
        name = upload.filename
	path = 'files/' + name
	if size < 2048:
		t = open(path, 'wb')
		t.write(content)
		t.close()
		f.write("<a href='%s'>%s</a><br>\\n" % (path, name))
        	print "Succesfully uploaded file <b>%s</b> (%d bytes).<br>" % (name, size)
        	print "<a href='PATH-TO-THE-SCRIPT'>See the file list or upload another file</a>"
	else:
		print "Maximum file size is 2 kB."
else:
	print f.read()
	print """<br>Upload your file:<br><form action="PATH-TO-THE-SCRIPT"
        method="post" enctype="multipart/form-data">
        <input type="file" name="upload">
        <input type="submit" value="Upload">
        </form>"""

f.close()
print '</body></html>'

Admire the script here.
As you can see, CGI in Python isn’t that difficult. All file data can be accessed via upload.file.(the data you want) or just upload.(the data you want). In the script I create the file using binary mode, so that images would be properly uploaded as well. Now, let’s get to something more advanced!

Your own CGI server

All that’s needed to set up a simple HTTP server lies in the module BaseHTTPServer. The following example show the simpliest HTTP server that will allow you to run .cgi files as scripts on your computer:

#!/usr/bin/python

import CGIHTTPServer, BaseHTTPServer

address = ('', 666)

server = BaseHTTPServer.HTTPServer(address, CGIHTTPServer.CGIHTTPRequestHandler)
print "Serving at 127.0.0.1:666..."
server.serve_forever()

The first parametre of the address list is the server address (e.g. 128.666.0.0), the second one is the port. If no address is given, the server works on localhost (http://127.0.0.1 or http://localhost). Printing the text will inform you that the server actually started working, if you access it via the command line. Remember to put server.serve_forever() at the end of the file to make the server work.
Now, enter http://127.0.0.1:666 in your browser and you should see the file list of the folder the script’s in. Create a directory, e.g. cgi-bin and a script inside it. Remember to give proper permitions - and that’s it! I can show you a screenshot.

The article’s showed you the basic CGI usage. Hope you enjoyed it and will make use of it. You can see how simple Python is; and it can do almost everything. I’ll write more articles on the usage of Python in different situations in the near future. C’ya!

posted on Tuesday, 14th August, 2007 at 16:08 (UTC +1000)
filed under Python, Tutorial, Web
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20 Responses to “[siː dʒiː aɪ]”

PiotrLegnica says:
August 15th, 2007 at 17:29

Why not WSGI? And why you haven’t made OpenID-enabled comments yet? :P

regua says:
October 12th, 2007 at 22:19

I’ll get to WSGI later.

I can’t find any suitable OpenID Wordpress plugin, and I really don’t think OpenID is worth writing a plugin by myself ;P

ychaocuhe says:
March 6th, 2008 at 16:35

if data.has_key(’upload’):
upload = data[’upload’]
content = upload.file.read()
size = len(content)

What happens if someone tries to upload a file of, say, 20Go ?

How about this :
if data.has_key(’upload’):
upload = data[’upload’]
upload.file.seek(0,2)
size = upload.file.tell()

This way you don’t have to read the entire file (and waste memory space) to know its size.

Y.chaouche

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