How to flash a message that user session was expired in flask

login_mgr = LoginManager(app)
login_mgr.login_view = 'login'
login_mgr.refresh_view = 'relogin'
login_mgr.needs_refresh_message = (u"Session timedout, please re-login")
login_mgr.needs_refresh_message_category = "info"

@app.before_request
def load_user():
    if "user_id" in session:
        g.user = db.session.get(session["user_id"])

@app.before_request
def before_request()

    now = datetime.datetime.now()
    try:
        last_active = session['last_active']
        delta = now - last_active
        if delta.seconds > 1800:
            session['last_active'] = now
            return logout('Your session has expired after 30 minutes, you have been logged out')
    except:
        pass

    try:
        session['last_active'] = now
    except:
        pass