I want to generate sequence number in purchase.requisition.line. Below is my code. Can anyone please correct me?

class purchase_requisition(osv.osv):
      _inherit = 'purchase.requisition'
      
      def update_seq(self, cr, uid, ids, line_ids, context=None):
          print "line_ids",line_ids
          new_order_line = []
          counter = 1
          x = {}
          for line in line_ids:
              if line[0] in [1,4]:
                 line[0] = 1
              if type(line[2]) == type({}):
                 line[2].update({'serial_no':counter})
                 print "counter1",counter
                 #x = counter
              else:
                 line[2] = {'serial_no':counter}
                 print "counter2",counter
                 #x = counter
              counter = counter + 1
              print "counter",counter
              x = new_order_line.append(line)
              print "new_order_line",new_order_line
          return {'value': {'line_ids': x} }
      
      

class purchase_requisition_line(osv.osv):
      _inherit = 'purchase.requisition.line'
      
      def update_seq(self, cr, uid, ids, line_ids, context=None):
          print "line_ids",line_ids
          new_order_line = []
          counter = 1
          for line in line_ids:
              if line[0] in [1,4]:
                 line[0] = 1
              if type(line[2]) == type({}):
                 line[2].update({'serial_no':counter})
                 print "counter1",counter
              else:
                 line[2] = {'serial_no':counter}
                 print "counter2",counter
              counter = counter + 1
              print "counter",counter
              new_order_line.append(line)
              print "new_order_line",new_order_line
          return {'value': {'serial_no': new_order_line} }

      _columns={
            'serial_no':fields.integer('Serial No'),
      }
      
      _order = 'serial_no desc, serial_no, id'
      _defaults = {'serial_no': 1,
                 }

purchase_requisition_line()

I didnt get the output.