首先，卸载干净：
rpm -qa|grep python|xargs rpm -e --allmatches --nodeps whereis python|xargs rm -fr rpm -qa|grep yum|xargs rpm -e --allmatches --nodeps rm -rf /etc/yum.repos.d/* whereis yum|xargs rm -fr
然后
cat /etc/redhat-release uname -a
查看系统版本，
然后去http://vault.centos.org/找到系统对应的版本号，找到url依次下载，
巨坑慎入：
wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-2.7.5-58.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-iniparse-0.4-9.el7.noarch.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-pycurl-7.19.0-19.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-devel-2.7.5-58.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-libs-2.7.5-58.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/python-urlgrabber-3.10-8.el7.noarch.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/rpm-python-4.11.3-25.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/yum-3.4.3-154.el7.centos.noarch.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/yum-metadata-parser-1.1.4-10.el7.x86_64.rpm wget http://vault.centos.org/7.4.1708/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.31-42.el7.noarch.rpm
安装：
rpm -ivh python-* rpm-python-* rpm -ivh yum-*
sudo vim /usr/bin/yum
把文件头部的#!/usr/bin/python改成#!/usr/bin/python2.7保存退出即可.
/usr/libexec/urlgrabber-ext-down看看，发下他也使用了/usr/bin/python，于是跟前面一样，改为2.7，完成。