scrapy随机更换user-agent

from fake_useragent import UserAgent
class (object):
    def __init__(self,crawler):
        super(RandomUserAgentMiddleware,self).__init__()
    def from_crawler(cls,crawler):
        return cls(crawler)
    def process_request(self,request,spider):
        # 调用fake-useragent  随机生成UA
        ua = UserAgent()
        request.headers.setdefault('User-Agent',ua.random)

DOWNLOADER_MIDDLEWARES = {
   'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': None,
   'lagou.middlewares.RandomUserAgentMiddleware': 500,
}