[mod_python] Problems with mod_python

I am having problems getting mod_python to work correctly.
I am following the directions in
http://www.modpython.org/live/mod_python-3.2.8/doc-html/inst-testing.html
and have written a mptest.py. However, when I access mptest.py from my browser,
the code does not execute and all I get is the text for mptest.py
displaying in the browser window. For example:

from mod_python import apache

def handler(req):
    req.content_type = 'text/plain'
    req.write("Hello World!")
    return apache.OK

Does anyone have any suggestions on what could be wrong with my installation
or how I can further diagnose this problem? Thanks in advance and I apologize
if this situation has been answered before.