Hi Nedas,

   Right now in onchange no need to return anything(in new api), but there is a case when we want to return some warning, domain or other stuff. So, in this case we can return as old api style like.

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@api.onchange('my_field')

def onchange_my_field(self):

     msg={}

     self.my_other_field = self.my_field #Here just copy whatever come with my_field into my_other_field

     if (self.warning==True):

         msg = { 'title': _('Waning!'), 'message' : 'You have just copy my field into my other field...'

                         }

   return {'warning': msg}

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same way we can return other stuff.

Hope this will help u as u want...