为什么req = urllib2.urlopen(url)会报错啊？而且只生成了一个无效图片

我的正则表达式应该没有错

import urllib2

import re 

req = urllib2.urlopen('https://www.imooc.com/course/list')

buf = req.read()

listurl = re.findall(r'src.*\.jpg',buf)

listurl = re.sub(r'src="','http:',str(listurl))

i = 0

for url in listurl:

    f = open('C:\\Users\\lenovo\\Desktop\\a\\'+str(i)+'.jpg','wb+')

    req = urllib2.urlopen(url)

    buf = req.read()

    f.write(buf)

    i = i + 1

print listurl