Centos自带python2，由于执行yum需要python2，所以即使安装了python3也不能删除python2
1.安装依赖包
yum -y groupinstall "Development tools"
yum -y install zlib-devel bzip2-devel openssl-devel ncurses-devel sqlite-devel readline-devel tk-devel gdbm-devel db4-devel libpcap-devel xz-devel libffi-devel
2.下载自己需要的python版本，例如python3.7.4,下载要花费一段时间，要耐心等待
wget https://www.python.org/ftp/python/3.7.4/Python-3.7.4.tar.xz
3.新建一个文件夹存放python3
mkdir /usr/local/python3
4.把安装包移动到该新建文件夹下，解压安装包，安装python3,依次执行以下命令，花费时间较长，耐心等待
mv Python-3.7.4.tar.xz /usr/local/python3
cd /usr/local/python3
tar -xvf Python-3.7.4.tar.xz
cd Python-3.7.4
./configure --prefix=/usr/local/python3 --enable-shared CFLAGS=-fPIC
make
make install
运行如果出问题
python3: error while loading shared libraries: libpython3.7m.so.1.0: cannot open shared object file: No such file or directory
yum install -y libffi-devel
cp /usr/local/python3/lib/libpython3.7m.so.1.0 /usr/lib
ldconfig
5.创建软连接
ln -s /usr/local/python3/bin/python3 /usr/bin/python3
ln -s /usr/local/python3/bin/pip3 /usr/bin/pip3
6.此时python3已经装好，在命令行中输入python3测试
7.修改yum配置文件，python2与python3共存
vi /usr/bin/yum
把#! /usr/bin/python修改为#! /usr/bin/python2（配置文件第一行）
同理
vi /usr/libexec/urlgrabber-ext-down
把文件里面的#! /usr/bin/python 也修改为#! /usr/bin/python2
此时完成python3安装，且实现与python2共存,保持yum命令可用
更改python环境变量的方式
1，echo $PATH
2，vi /etc/profile
3, PATH=$PATH:/usr/local/python3/bin（在最后一行写入）
4, :wq！(退出)
5，source /etc/profile