请问这个代码为什么会出现urllib2.HTTPError: HTTP Error 400: Bad Request错误？

Question

代码如下：

import re

import urllib2

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

buf = req.read()

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

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

#print listurl

i = 0

for url in listurl:

    f=open("D:\\python\\LearnPython\\"+str(i)+'.jpg','wb')

    req = urllib2.urlopen('http:'+url)

    buf = req.read()

    f.write(buf)

    i=i+1