sudo apt-get update连接失败问题解决

在安装python-pip时出现连接失败错误

进而在执行sudo apt-get update是也出现连接失败错误，怀疑是源不可用：

搜索得知如下解决方案：
1. 先关闭所有源

2. rm -rf /var/lib/apt/lists/*删除/var/lib/apt/lists 所有文件
3. 修改/etc/apt/sources.list文件。将源改成163的或者阿里的：
（注意版本匹配：16.1->yakkety;16.04->xenial;15.10->willy;14.04->trusty;12.04->precise）

deb http://mirrors.163.com/ubuntu/ xenial main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ xenial-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ xenial main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ xenial-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ xenial-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ xenial-backports main restricted universe multiverse

4.再次sudo apt-get update成功

参考：
ubuntu sudo apt-get update 失败 解决方法
ubuntuapt-getupdate失败解决方案
ubuntu16.04更换为国内源