Python使用镜像下载安装包

以使用PyQt5为例：
镜像下载：
pip install PyQt5 -i https://pypi.douban.com/simple
报错：
ERROR: Could not install packages due to an EnvironmentError: [WinError 5] 拒绝访问。: ‘c:\program files\python37\Lib\site-packages\PyQt5’
Consider using the --user option or check the permissions.
继续：错误原因：权限不够！
pip install PyQt5 --user -i https://pypi.douban.com/simple

镜像下载的原理？