After an item has been scraped by a spider,it is sent to the Item Pipeline which process it through several components that are executed sequentially.
Each item pipeline component is a single python class that must implement the following method:
process_item(item,spider)
This method is called for every item pipeline component and must either return a Item object or raise a DropItem exception.Dropped items are no longer processed by further pipeline component.
参数：item(Item object) ——the item scraped
spider(Spider object)——the spider which scraped the item