python安装requests报错Cannot fetch index base URL

root@sd-63877:/home# pip install requests
Downloading/unpacking requests
Cannot fetch index base URL http://pypi.python.org/simple/
Could not find any downloads that satisfy the requirement requests
No distributions at all found for requests
Storing complete log in /root/.pip/pip.log

解决：

pip install requests -i https://pypi.python.org/simple

pip install requests -i https://pypi.douban.com/simple