Centos自帶python2，由於執行yum需要python2，所以即使安裝了python3也不能刪除python2
1.安裝依賴包
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.下載自己需要的python版本，例如python3.7.4,下載要花費一段時間，要耐心等待
wget https://www.python.org/ftp/python/3.7.4/Python-3.7.4.tar.xz
3.新建一個文件夾存放python3
mkdir /usr/local/python3
4.把安裝包移動到該新建文件夾下，解壓安裝包，安裝python3,依次執行以下命令，花費時間較長，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
運行如果出問題
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.創建軟連接
ln -s /usr/local/python3/bin/python3 /usr/bin/python3
ln -s /usr/local/python3/bin/pip3 /usr/bin/pip3
6.此時python3已經裝好，在命令行中輸入python3測試
7.修改yum配置文件，python2與python3共存
vi /usr/bin/yum
把#! /usr/bin/python修改爲#! /usr/bin/python2（配置文件第一行）
同理
vi /usr/libexec/urlgrabber-ext-down
把文件裏面的#! /usr/bin/python 也修改爲#! /usr/bin/python2
此時完成python3安裝，且實現與python2共存,保持yum命令可用
更改python環境變量的方式
1，echo $PATH
2，vi /etc/profile
3, PATH=$PATH:/usr/local/python3/bin（在最後一行寫入）
4, :wq！(退出)
5，source /etc/profile