Introduction to Programming (Python) (IPP) CGI Programming. $Date: 2010/11/25 09:18:11 $ IPP-15 1

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1 Introduction to Programming (Python) (IPP) CGI Programming $Date: 2010/11/25 09:18:11 $ 1

2 Static web pages The simplest web-based interaction is when your browser (the client) gets sent a file encoded in HTML HTML = hypertext markup language The browser decodes the HTML and you see the finished product displayed. Let s look at some HTML... 2

3 CGI: Common Gateway Interface A CGI script is a program that is executed on the server. The output of the CGI script (generally HTML) is sent to the browser (rather than a static file). As long as your server has Python installed you can write CGI scripts in Python. 3

4 Hello world revisited CGI scripts don t have to be complicated! The header (and the newlines) are needed by the browser. #!/usr/bin/env python print "Content-Type: text/html\n\n" print hello world # HTML coming! 4

5 Installing CGI scripts I put the file hello.py in my cgi-bin directory on the CS web server. The first line makes it an executable as long as I ve remembered to do chmod +x hello.py 5

6 Running CGI scripts It s enough to point your web browser at to get hello world to appear on your web browser. Because of where it is, and because it is executable, the server knows to execute the file rather than just display it. 6

7 Installing CGI scripts on biolarfs1 Put your scripts in ~/public_html/cgi-bin On biolarfs1 the name of your script matters The name has to end in.cgi 7

8 Running CGI scripts on biolarfs1 I saved hello.cgi in ~jc33/public_html/cgi-bin To run it point your browser at: biolarfs1 is only reachable from york.ac.uk 8

9 Sending variables to CGI scripts We need a way of sending input to CGI scripts user=james&greeting=hi calls the script hellowho.py with the variable user set to james and the variable greeting set to hi Hmmm, how does the script get hold of these values? 9

10 Using the cgi module to get values Here s hellowho.py #!/usr/bin/env python import cgi import cgitb; cgitb.enable() print "Content-Type: text/html\n\n" # solely for debugging # HTML coming! form = cgi.fieldstorage() # the only object you need! print form[ greeting ].value, form[ user ].value 10

11 CGI debugging CGI script debugging can be tricky, since the program runs on the web server, not your machine. Always add the import cgitb; cgitb.enable() line. It generates exceptionally useful debugging information. If you don t, you will get very little helpful information. 11

12 Getting information from forms Usually CGI scripts get information from forms into which users paste information. Let s look at submit code.html Note: submit code.html is static HTML it is not a CGI script 12

13 What CGI scripts can do for you Apart from the unusual input (usually via forms), and the unusual output (often HTML sent back to the browser), Python CGI scripts can do anything other Python scripts can do... do computations, interact with databases, fire off s etc, etc 13

14 Generating HTML (from CGI scripts) The simplest crude option is just to print out raw HTML, perhaps with a bit of string processing for dynamic content Let s look at hellolist.py 14

15 Simple HTML generation from template files Another fairly crude option is for your CGI script to edit a template HTML file. Let s look at template.html, and its sneaky using comments trick! 15

16 Simple editing for dynamic HTML maketimetable.py uses re.sub to do the editing. 16

17 Bigger web projects Bigger web projects (Python or otherwise) build on existing code. Zope is an open source application server for building content management systems, intranets, portals, and custom applications. Zope is said to be Python s killer-app. Basically, you publish Python objects on the web. 17

18 Plone Plone ( is a free content management system. I was on a project that uses icoya (built on top of Plone and Zope). Life s too short to do big web applications from scratch! 18

19 Django I m a fan of Django ( I used it to implement the CS dept s project allocation system. 19