我不确定这是问题的直接原因.但是表单发送到登录操作还有另一个参数.

I'm not sure this is direct reason for the problem. But there are another parameter that the form send to login action.

除了username，password，还要传递以下参数:

Beside username, password, also pass following parameters:

'username': username,
'password': password,
'login': 'Login'.
'sid': sid,   # You need to parse the login page to get sid
'redirect': 'index.php',