if request.session.get('is_login',None):
   return HttpResponseRedirect(reverse('index'))

request.session['is_login'] = True
request.session['user_id'] = db_user.id
request.session['user_name'] = db_user.name

def logout(request):
  if not request.session.get('is_login', None):
    # 如果本来就未登录，也就没有登出一说
    return redirect("/index/")
  request.session.flush()
  # 或者使用下面的方法
  # del request.session['is_login']
  # del request.session['user_id']
  # del request.session['user_name']
  return redirect("/index/")

{% if request.session.is_login %}
   <li><a href="#" rel="external nofollow" >当前在线：{{ request.session.user_name }}</a></li>
   <li><a href="/logout/" rel="external nofollow" >登出</a></li>
 {% else %}
   <li><a href="/login/" rel="external nofollow" >登录</a></li>
   <li><a href="/register/" rel="external nofollow" >注册</a></li>
{% endif %}