Using Python to sign into website, fill in a form, then sign out

import urllib
import urllib2

name =  "name field"
data = {
        "name" : name 
       }

encoded_data = urllib.urlencode(data)
content = urllib2.urlopen("http://www.abc.com/messages.php?action=send",
        encoded_data)
print content.readlines()

just replace http://www.abc.com/messages.php?action=send with the url where your form is being submitted

reply to your comment: if the url is the url where your form is located, and you need to do this just for one website, look at the source code of the page and find

<form method="POST" action="some_address.php">

and put this address as parameter for urllib2.urlopen

And you have to realise what submit button does.
It just send a Http request to the url defined by action in the form.
So what you do is to simulate this request with urllib2