方法1：使用build_opener()来添加
import urllib2
url = "https://www.baidu.com"
headers = ("User-Agent","Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36")
opener = urllib2.build_opener()
opener.addheaders = [headers]
file = opener.open(url)
html = file.read()
print html
方法2：利用add_header()来添加
import urllib2
url = "https://blog.51cto.com/lsfandlinux/2046467"
req = urllib2.Request(url)
req.add_header("User-Agent","Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36")
file = urllib2.urlopen(req)
html = file.read()
print html