scrapy中为什么用到yield

>>> from urllib import parse
>>> url="https://www.baidu.com/posts"
>>> post_url = 'n/222'
>>> parse.urljoin(url,post_url)
'https://www.baidu.com/n/222'
>>> post_url = "https://www.baidu.com/posts/n/222"
>>> parse.urljoin(url,post_url)
'https://www.baidu.com/posts/n/222'