How to change default python versions on fedora 27 / CentOS / RHEL ?

How to change default python versions on fedora 27 / CentOS / RHEL ?

how to we know the available python version packages on fedora linux system?

$ ls /usr/bin/python*

Lists the available python version packages as below.

/usr/bin/python2	/usr/bin/python2.7-futurize	/usr/bin/python3	/usr/bin/python3.6m
/usr/bin/python2.7  /usr/bin/python2.7-pasteurize  /usr/bin/python3.6  /usr/bin/python3-chardetect

Based on above, there are two python versions python 2.7 and python3.6 available in fedora system.

How to check the python default version in fedora

$ python -V

Python 2.7.15

By default python3 command is there to run the python3 code on fedroa system.

$ python3 -V
Python 3.6.6

How to change python command to use python3 version for current user on fedora system ?

$ alias python='/usr/bin/python3.6'

Now default version for python command is using python3.6 version for current user.

$ python -V
Python 3.6.6

but if you check the python default version in other user, this change will not be updated.

Let's check with root user

$ sudo -i
# python -V
Python 2.7.15

How to change python defualt versions globally on fedora / centos / RHEL linux ?

alternatives command is available in fedora to change the python version globally to all the users on fedara linux system.

alternatives syntax

   	alternatives --install    
                	[--initscript ]
                	[--family ]
                	[--slave   ]*
   	alternatives --remove  
   	alternatives --auto 
   	alternatives --config 
   	alternatives --display 
   	alternatives --set  
   	alternatives --list
   	alternatives --remove-all 

Let us first check whether any alternative python version is registered in the fedora system

$  alternatives --list | grep -i python

If no alternatives registered, nothing will be displayed.

To register the python alternatives, need to use alternatives with --intall option.

Creates the alternative python version for 3.6 with high priority 10

$ sudo  alternatives --install /usr/bin/python python /usr/bin/python3.6 10

Creates the alternative python version for 2.7 with priority 1.

$ sudo alternatives --install /usr/bin/python python /usr/bin/python2.7 1

Once created alternative python versions, we can change the python version using alternatives command with --config option.

$ sudo alternatives --config python

There are 2 programs which provide 'python'.

  Selection	Command
-----------------------------------------------
*+ 1       	/usr/bin/python3.6
   2       	/usr/bin/python2.7

Enter to keep the current selection[+], or type selection number: 2

Here we have selected the python version 2.7, and check the default python version in fedora linux

$ python -V
Python 2.7.15

Check the python version as root user as well.

$ sudo -i
# python -V
Python 2.7.15