Connexion Steam mechanize browser

import mechanize
import cookielib

jar = cookielib.LWPCookieJar()
br = mechanize.Browser()
cookiejar = cookielib.LWPCookieJar() 
br.set_cookiejar(cookiejar)
br.set_handle_robots(False)
br.set_handle_redirect(True)
br.addheaders = [('User-agent','Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1500.72 Safari/537.36')]
br.set_handle_refresh(mechanize._http.HTTPRefreshProcessor(), max_time=1)

USERNAME = "myusername"
PASSWORD = "mypassword"

f = open("output.html", "w")
br.open("https://steamcommunity.com/login/home/")
br.select_form(nr=0)  #la bonne form d'apres moi
br.form['username'] = USERNAME
br.form['password'] = PASSWORD
br.submit()
f.write(br.response().read())
f.close()