cost_price = fields.Float('Cost Price', digits=(16, 4), states={
                'invisible': Or(~Eval('_parent_invoice', {}).get('type').in_(['out_credit_note']), Eval('type') != 'line'),
                'required': And(Eval('_parent_invoice', {}).get('type').in_(['out_credit_note']),
                                Eval('type') == 'line',
                                Eval('_parent_invoice', {}).get('state', 'draft') != 'draft')