OdooModel.py

class CustomPosOrder(models.Model):
    _inherit = 'pos.order'
    pass
  
  # Quiero implementar sobreescribir el metodo aqui en lugar de en   CustomLegacyPosOrder

class CustomLegacyPosOrder(osv.osv):
    """
        Made for overriding the odoo 8 pos.order _amount_all method, so that legal tip is included in total
    """
    _name = 'pos.order'
    _inherit = 'pos.order'

    def _amount_all(self, cr, uid, ids, name, args, context=None):
        res = super(CustomLegacyPosOrder, self)._amount_all(cr, uid, ids, name, args, context)
        for order in self.browse(cr, uid, ids, context=context):
            amount_untaxed = 0
            for line in order.lines:
                amount_untaxed += line.price_subtotal

            cr.execute('SELECT using_legal_tip FROM pos_order WHERE id = ' + str(order.id))
            using_legal_tip = cr.fetchone()[0]

            print 'Total Amount: ' + str(res[order.id]['amount_total'])

            if using_legal_tip:
                res[order.id]['amount_total'] += amount_untaxed * 0.10

            print 'Total Amount With Legal Tip: ' + str(res[order.id]['amount_total'])

        return res