It had been reported that changing the default python version with update-alternatives breaks the shell, so use it at your own risk!

Recently my team has stopped supporting python 2.7 and only supports python version 3.5 and above. For testing purposes, I had to install all three (at the time of writing this post) 3.5, 3.6, 3.7, 3.8, and 3.9 in the same system. But it was a bit of a hassle to delete and create a new link for /usr/bin/python3 and sometimes it messes things up.

To be on the safe side, I have spent a bit of time testing different methods of switching between python versions. There are a couple of different ways to do this but the following is the one I found easy to use and had no apparent side effects (in my case) on the system.

NOTE Tested on ubuntu 16.04, 18.04, 20.04

Once all three versions of python is installed. It’s time to configure the alternatives.

Add Python Versions to Update Alternatives

sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python3.6 2
sudo update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python3.7 3
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 4
sudo update-alternatives --install /usr/bin/python3 python3 /usr/local/bin/python3.9 5

/usr/local/bin/python3.6 or /usr/local/bin/python3.9 is the full path to the python binary and 1, 2, 3, 4 and 5 at the end is the priority level.

sudo update-alternatives --config python3

Choose a version and put the digit as a selection number and press ENTER.

Check Python Version

Now you can check if the python3 is pointing to the right version or not by using the following command

python3 --version

Enjoy!