在实验操作时直接用yum remove python导致yum没法用；
或者是安装python3.5后，将/usr/bin/python 软连接改成python3后，yum命令报错；
常用恢复法：
一、针对误卸载yum
步骤1：卸载python
rpm -qa|grep python|xargs rpm -e --allmatches --nodeps
whereis python|xargs rm -frrpm -qa|grep yum|xargs rpm -e --allmatches --nodeps
rm -rf /etc/yum.repos.d/*
whereis yum|xargs rm -frmkdir /usr/local/src/python
cd /usr/local/src/python
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-2.7.5-58.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-iniparse-0.4-9.el7.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-pycurl-7.19.0-19.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-devel-2.7.5-58.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-libs-2.7.5-58.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/python-urlgrabber-3.10-8.el7.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/rpm-python-4.11.3-25.el7.x86_64.rpm
rpm -ivh python-* rpm-python-*mkdir /usr/local/src/yum
cd /usr/local/src/yum
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-3.4.3-154.el7.centos.noarch.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
wget http://mirrors.163.com/centos/7/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-42.el7.noarch.rpm
rpm -ivh yum-*
测试 yum
二、针对装了python3后使用yum报错
解决办法1.不要修改软链接，使用python3命令
解决办法2.实在需要不要修改软链接，修改/usr/bin/yum
#!/usr/bin/python 改为 #!/usr/bin/python2.7
解决。
三、更改yum源与更新系统
[1] 首先备份/etc/yum.repos.d/CentOS-Base.repo
cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
[2] 进入yum源配置文件所在文件夹
[root@localhost yum.repos.d]# cd /etc/yum.repos.d/
[3] 下载163的yum源配置文件，放入/etc/yum.repos.d/(操作前请做好相应备份)
[root@localhost yum.repos.d]# wget http://mirrors.163.com/.help/CentOS7-Base-163.repo
[4] 运行yum makecache生成缓存
[root@localhost yum.repos.d]# yum makecache
[5] 更新系统(时间比较久,主要看个人网速)
[root@localhost yum.repos.d]# yum -y update
[6] 安装vim编辑器
。。。