@Prince,

Here you go!

 @api.onchange(<your field name>)
    def _onchange_student(self):
       body of your code

#Note : in odoo v8 you no need to call that method from xml file it will directly call when you do changes in field which you have passed as argument in @api.onchange method. it will call.

Hope this will helps you.

Regards,

Anil.