Using Python and Mechanize to submit form data and authenticate

I would definitely suggest trying to use the API if possible, but this works for me (not for your example post, which has been deleted, but for any active one):

#!/usr/bin/env python

import mechanize
import cookielib
import urllib
import logging
import sys

def main():

    br = mechanize.Browser()
    cj = cookielib.LWPCookieJar()
    br.set_cookiejar(cj)

    br.set_handle_equiv(True)
    br.set_handle_gzip(True)
    br.set_handle_redirect(True)
    br.set_handle_referer(True)
    br.set_handle_robots(False)

    br.set_handle_refresh(mechanize._http.HTTPRefreshProcessor(), max_time=1)

    r= br.open('http://www.reddit.com')

    # Select the second (index one) form
    br.select_form(nr=1)

    # User credentials
    br.form['user'] = 'user'
    br.form['passwd'] = 'passwd'

    # Login
    br.submit()

    # Open up comment page
    posting = 'http://www.reddit.com/r/PoopSandwiches/comments/f47f8/testing/'
    rval="PoopSandwiches"
    # you can get the rval in other ways, but this will work for testing

    r = br.open(posting)

    # You need the 'uh' value from the first form
    br.select_form(nr=0)
    uh = br.form['uh']

    br.select_form(nr=7)
    thing_id = br.form['thing_id']
    id = '#' + br.form.attrs['id']
    # The id that gets posted is the form id with a '#' prepended.

    data = {'uh':uh, 'thing_id':thing_id, 'id':id, 'renderstyle':'html', 'r':rval, 'text':"Your text here!"}
    new_data_dict = dict((k, urllib.quote(v).replace('%20', '+')) for k, v in data.iteritems())

    # not sure if the replace needs to happen, I did it anyway
    new_data="thing_id=%(thing_id)s&text=%(text)s&id=%(id)s&r=%(r)s&uh=%(uh)s&renderstyle=%(renderstyle)s" %(new_data_dict)

    # not sure which of these headers are really needed, but it works with all
    # of them, so why not just include them.
    req = mechanize.Request('http://www.reddit.com/api/comment', new_data)
    req.add_header('Referer', posting)
    req.add_header('Accept', ' application/json, text/javascript, */*')
    req.add_header('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8')
    req.add_header('X-Requested-With', 'XMLHttpRequest')
    cj.add_cookie_header(req)
    res = mechanize.urlopen(req)

main()

It would be interesting to turn javascript off and see how the reddit comments are handled then. Right now there is a bunch of magic that happens in an onsubmit function called when making your post. This is where the uh and id value get added.