@frontend.route("/login/action/",methods=['POST'])
def logging_in():
    username = request.form.get("username")
    password = request.form.get("password")
    if username and password:
        user = db.user.User.one({'username':username,'password':password})
        if user:
            session['logged_in'] = True
            g.user = user
            flash('Welcome Back %s' % user['username'],'info')
            return url_for("start")
    flash('Access Denied','error')
    return url_for("login")

@frontend.route("/start/")
def start():
    print session['logged_in'] # tutto ok!!
    print g.user # ERRORE!!!
    return render_template("start.html")