(ClientCookie is a module for (automatic) cookie-handling: http://wwwsearch.sourceforge.net/ClientCookie)

# I encode the data I'll be sending:
data = urllib.urlencode({'username': 'mandark', 'password': 'deedee'})

# And I send it and read the page:
page = ClientCookie.urlopen('http://www.forum.com/ucp.php?mode=login', data)
output = page.read()

The script doesn't log in, but rather seems to get redirected back to the same login page asking it for a username and password. What am I doing wrong?

Any help would be greatly appreciated! Thanks!

Have you tried fetching the login page first?

I would suggest using Tamper Data to have a peek at exactly what's being sent when you request the login page and then log in normally using a web browser from a fresh start, with no initial cookies in place, so that your script can replicate it exactly.

That's the approach I used when writing the following, extracted from a script which needs to login to an Invision Power Board forum, using cookielib and urllib2 - you may find it useful as a reference.

import cookielib
import logging
import sys
import urllib
import urllib2

cookies = cookielib.LWPCookieJar()
opener = urllib2.build_opener(urllib2.HTTPCookieProcessor(cookies))
urllib2.install_opener(opener)
headers = {
    'User-Agent': 'Mozilla/5.0 (Windows; U; Windows NT 5.0; en-GB; rv:1.8.1.12) Gecko/20080201 Firefox/2.0.0.12',
    'Accept': 'text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5',
    'Accept-Language': 'en-gb,en;q=0.5',
    'Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.7',
}

# Fetch the login page to set initial cookies
urllib2.urlopen(urllib2.Request('http://www.rllmukforum.com/index.php?act=Login&CODE=00', None, headers))

# Login so we can access the Off Topic forum
login_headers = headers.copy()
login_headers.update({
    'Referer': 'http://www.rllmukforum.com/index.php?act=Login&CODE=00',
    'Content-Type': 'application/x-www-form-urlencoded',
})
html = urllib2.urlopen(urllib2.Request('http://www.rllmukforum.com/index.php?act=Login&CODE=01',
                                       urllib.urlencode({
                                           'referer': 'http://www.rllmukforum.com/index.php?',
                                           'UserName': RLLMUK_USERNAME,
                                           'PassWord': RLLMUK_PASSWORD,
                                       }),
                                       login_headers)).read()
if 'The following errors were found' in html:
    logging.error('RLLMUK login failed')
    logging.info(html)
    sys.exit(1)

I'd recommend taking a look at the mechanize library; it's designed for precisely this type of task. It's also far easier than doing it by hand.