Step-1 Install the purchase_usability_extension module.
Step-2 purchase_usability_extension->Purchases.py
Step-3 After that custmozie purchases.py(Python file) file:
In below example, I have custmozied purchases.py file. You can directly copy the below python code in your purchases.py file.

from openerp import models, fields


class PurchaseOrderLine(models.Model):
    _inherit = 'purchase.order.line'
    _order = 'order_id, sequence, id'

    sequence = fields.Integer(string='Sequence', default=10)


class PurchaseOrder(models.Model):
    _inherit = 'purchase.order'

    state = fields.Selection(track_visibility='onchange')
    location_id = fields.Many2one(track_visibility='onchange')
    dest_address_id = fields.Many2one(track_visibility='onchange')
    pricelist_id = fields.Many2one(track_visibility='onchange')
    date_approve = fields.Date(track_visibility='onchange')
    validator = fields.Many2one(track_visibility='onchange')
    invoice_method = fields.Selection(track_visibility='onchange')
    payment_term_id = fields.Many2one(track_visibility='onchange')
    fiscal_position = fields.Many2one(track_visibility='onchange')
    incoterm_id = fields.Many2one(track_visibility='onchange')
    partner_ref = fields.Char(track_visibility='onchange')