I am using sequence to let user  manually  change the presentation of data (property_type) , my python code is:

_order = "sequence, property_type"

sequence = fields.Integer('Sequence', default=1, help="Used to order stages. Lower is better.")
property_type = fields.Selection(string='Property Type1', required=True, \
selection=[('house', 'House'), ('apartment', 'Apartment'), ('penthouse', 'Penthouse'), ('castle', 'Castle')])
and my xml code (I don't know why each time I paste xml code, here it will get distorted or even missing so  I  enclosed  them  in  remark  tag ):