On Puchase.py

 def make_po(self, cr, uid, ids, context=None):

.......
 available_draft_po_ids = po_obj.search(cr, uid, [

('partner_id', '=', partner.id), ('state', '=', 'draft'), ('picking_type_id', '=', procurement.rule_id.picking_type_id.id),
('location_id', '=', procurement.location_id.id), ('company_id', '=', procurement.company_id.id), ('dest_address_id', '=', procurement.partner_dest_id.id)], context=context) 
 
......

i want set value of available_draft_po_ids is null but i don't want modify base code on purchase.py

So how to override it?

(sorry i speak English very bad)