老师您好，在用downloadmiddleware随机更换user-agent时我定义了函数RandomUserAgentMiddlware，并在settings文件中配置了，但是调试时程序只执行了__init__函数，没有调用process_request函数，不知道是什么原因

class RandomUserAgentMiddlware(object):
    #随机更换user-agent
    def __init__(self, crawler):
        super(RandomUserAgentMiddlware, self).__init__()
        self.ua = UserAgent()
        self.ua_type = crawler.settings.get("RANDOM_UA_TYPE", "random")

    @classmethod
    def from_crawler(cls, crawler):
        return cls(crawler)

    def process_request(self, request, spider):
        def get_ua():
            return getattr(self.ua, self.ua_type)

        request.headers.setdefault('User-Agent', get_ua())
        
SPIDER_MIDDLEWARES = {
    'MyArticleSpider.middlewares.RandomUserAgentMiddlware': 1,
    'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': None, #需要取消
}