How scrapy passes parameters between Requests

class HelloSpider(BaseSpider):    
    name = 'hellospider'    
    start_urls = (    
        'http://aaa.com/page1',    
        'http://aaa.com/page2',    
        )    
    
    def parse(self, response):    
        for item_url in item_urls:    
            yield Request(url=item_url, callback=self.parse_item)    
    
    def parse_item(self, response):    
        item = MyItem()    
        #通过meta={'item': item}传递参数
        yield Request(url=item_details_url, meta={'item': item},    
            callback=self.parse_detailsinfo)    
    
    def parse_detailsinfo(self, response):    
        #通过response.meta['item']获取参数
        item = response.meta['item']    
        return item