我必须在 ubuntu 12.04 上安装 virtualenv 但它显示以下错误
sudo pip install virtualenv
[sudo] password for raushan:
Downloading/unpacking virtualenv
Cannot fetch index base URL http://pypi.python.org/simple/
Could not find any downloads that satisfy the requirement virtualenv
No distributions at all found for virtualenv
Storing complete log in /home/raushan/.pip/pip.log