ValueError: unknown url type: '['

import urllib.request,re
req=urllib.request.urlopen('https://www.imooc.com/course/list')
buf=req.read().decode('utf-8')
listurl=re.findall(r'src.+.jpg',buf)
list_url=re.sub(r'src="','http:',str(listurl))
i=0
for url in list_url:
    f=open(str(i)+'.jpg','wb')
    req=urllib.request.urlopen(url)
    buf=req.read()
    f.write(buf)
    i=i+1
    f.close

第9行报错ValueError: unknown url type: '['  是什么情况呢