def process_request(self, request, spider):
    ua = random.choice(self.user_agent_list)
if ua:
    request.headers.setdefault('User-Agent', ua)
spider.log('User-Agent: {} {}'.format(request.headers.get('User-Agent'),
    request), level=log.DEBUG)
user_agent_list = [
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/22.0.1207.1 Safari/537.1'
    ,
    'Mozilla/5.0 (X11; CrOS i686 2268.111.0) AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11'
    ,
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/536.6 (KHTML, like Gecko) Chrome/20.0.1092.0 Safari/536.6'
    ]
How to make Scrapy show user agent per download request in log?