centos7 update python2 to python3

After linux centos7, it is necessary to upgrade python2.7 to python3 installation. Because most applications still use pytho3.6. In centos, 2.7 yum should be reserved.
Old python version: 2.7.5

New python version: 3.6.8 Less
talk, start:

Please install openssl before starting to install python3. Otherwise, verification error occurs when pip3 installs virtualenv.yum install openssl openssl-devel

1. From the Python official website to get the Python3 package, switch to the directory / usr / local / src and
   
   execute the command line:wget https://www.python.org/ftp/python/3.6.8/Python-3.6.8.tgz
   

2. Use the command, the following command to decompress:
   tar -xzf Python3.6.8.tgz
   

3. Create a directory under the / usr / local path – python3
   mkdir /usr/local/python3

4. Compile and install
   A. Execute command: cd /usr/local/src/Python-3.6.8
   
   B. Execute command: ./configure --prefix=/usr/local/python3
   
   C. Execute command:make all

   D. Execute the command: make install

5. Enter the absolute path of the installation and check whether the installation is successful.
   Run the command:/usr/local/python3/bin/python3.6 -V
   

6. Check the environment variables, and search by default in the order of PATH when starting python.
   Excuting an order:echo $PATH
   

7. Modify the soft connection, point to python3.6 when starting python
   A. Python
   execute command before backup : mv / usr / bin / python /usr/bin/python2.7.5 (please check your own python version first)
   
   B. Modify the soft connection
   Execute command: ln -s / usr / local / python3 / bin / python3 / usr / bin / python
   
   C. View python version:
   execute command:python
   

8. In order to input only when it is convenient, you can add alias
   A. execute command: cd ~
   B. execute command: cat .bashrc
   C. execute command:source .bashrc
   

9. Check whether python3 is started by default to
   execute the command:python3 -V
   

10. Modify the yum file, because after upgrading the version, yum will report an error:
    execute the command: sudo vim /usr/bin/yum
    
    follow the old version number after the first line!
    

11. Next, an error will be reported when installing the software:
    
    Solution:
    Execute the command: sudo vim /usr/libexec/urlgrabber-ext-down
    In the first line of the file, also change to python2.7 (2.7 is the version, change it according to your own version!)

12. Related pip3
    ln -s /usr/local/python3/bin/pip3 /usr/bin/pip3
    check version
    pip3 -V

Note that pip3 is included when compiling python3. If not, please use make when compiling python3

12. Use pip3 to install virtualenv.
    Before using pip3 to install virtualenv, first install openssl. Otherwise, it will prompt an error. After
    yum install openssl openssl-devel
    installing openssl and recompiling python3, install virtualenv.
    pip3 install virtualenv

13. Also, if there is a garbled situation when pressing the backspace key (or arrow keys),
    yum install readline-devel readline
    then recompile.