使用fake-useragent：
https://github.com/hellysmile/fake-useragent
这是一个可以随机切换访问头的插件

安装方法：

pip install fake-useragent

使用方法：

from fake_useragent import UserAgent
ua = UserAgent()

ua.ie
# Mozilla/5.0 (Windows; U; MSIE 9.0; Windows NT 9.0; en-US);
ua.msie
# Mozilla/5.0 (compatible; MSIE 10.0; Macintosh; Intel Mac OS X 10_7_3; Trident/6.0)'
ua['Internet Explorer']
# Mozilla/5.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; GTB7.4; InfoPath.2; SV1; .NET CLR 3.3.69573; WOW64; en-US)
ua.opera
# Opera/9.80 (X11; Linux i686; U; ru) Presto/2.8.131 Version/11.11
ua.chrome
# Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.2 (KHTML, like Gecko) Chrome/22.0.1216.0 Safari/537.2'
ua.google
# Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/537.13 (KHTML, like Gecko) Chrome/24.0.1290.1 Safari/537.13
ua['google chrome']
# Mozilla/5.0 (X11; CrOS i686 2268.111.0) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11
ua.firefox
# Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:16.0.1) Gecko/20121011 Firefox/16.0.1
ua.ff
# Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1
ua.safari
# Mozilla/5.0 (iPad; CPU OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A5355d Safari/8536.25

# and the best one, random via real world browser usage statistic
ua.random

写一个随机更换user-agent的Downloader Middleware，新的middlewares.py代码如下：

from fake_useragent import UserAgent
class RandomUserAgentMiddlware(object):
   
    def __init__(self, crawler):
        super(RandomUserAgentMiddlware, self).__init__()
        self.ua = UserAgent()
        #读取在settings文件中的配置，来决定ua采用哪个方法，默认是random，也可是ie、Firefox等等，参考前面的使用方法。
        self.ua_type = crawler.settings.get("RANDOM_UA_TYPE", "random")

    @classmethod
    def from_crawler(cls, crawler):
        return cls(crawler)
    
    #更换用户代理逻辑在此方法中
    def process_request(self, request, spider):
        def get_ua():
            return getattr(self.ua, self.ua_type)

        print  get_ua()
        request.headers.setdefault('User-Agent', get_ua())

最后，在settings.py中开启Middleware，同时还要关闭scrapy自带的代理Middleware：

# Enable or disable downloader middlewares
# See http://scrapy.readthedocs.org/en/latest/topics/downloader-middleware.html
DOWNLOADER_MIDDLEWARES = {
   'JobSpider.middlewares.RandomUserAgentMiddlware': 543,
   'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': None,
}