问题的出现
在使用pip安装python包时，出现以下错误：
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pelican/
WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pelican/
WARNING: Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pelican/
WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pelican/
WARNING: Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError(0, 'Error'))': /simple/pelican/
ERROR: Could not find a version that satisfies the requirement pelican (from versions: none)
ERROR: No matching distribution found for pelican
WARNING: There was an error checking the latest version of pip.
解决
该问题一般系代理导致，有两种解决办法。 1、关闭代理。 2、配置pip使用代理。
第一种方法不必赘述。第二种方法如下： 对于 linux 环境
export http_proxy=http://proxyAddress:proxyPort
export https_proxy=http://proxyAddress:proxyPort
对于 windows 下CMD环境
set http_proxy=http://proxyAddress:proxyPort
set https_proxy=http://proxyAddress:proxyPort
对于 windows 下PowerShell环境
$env:http_proxy="http://proxyAddress:proxyPort"
$env:https_proxy="http://proxyAddress:proxyPort"
完成后再次执行 pip install 即可。