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 reimport urllib2req = 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 listurli = 0for 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