Switching between Python 2 and 3 versions on Ubuntu 20.04

In the recent past, there has been a bit of debate on which Python version should one learn. Python 2 or Python 3. It’s now 2020 and pretty evident that Python 3 is the way to go. However, Python 2 is still in the market, and some people, including me, still use it in development. It brings forth the necessity of how we set up and switch between the two versions on your Linux system.

Ubuntu 18.04 LTS release came with several changes, and one is that Python 2 was no longer the default Python version in the system. Welcome, Ubuntu 20.04 LTS release; canonical dropped Python 2 entirely and no longer included it among the installation packages.

See the image below. When we try to execute the Python 2 command, which is by default “python,” we get “command not found” error.

Python 2 not installed in Ubuntu 20.04
Python 2 not packaged in Ubuntu 20.04

In this post, we will show you how to:

  • Install Python 2 in Ubuntu 20.04 LTS
  • Switch between Python 3 and Python 2 versions

Installing Python 2 in Ubuntu 20.04 LTS

Step 1) Launch the Terminal and type any of the commands below. You will be required to enter your root password.

$ sudo apt install python2
OR
$ sudo apt install python-minimal

Install Python2 in Ubuntu 20.04 LTS
Install Python2 in Ubuntu 20.04 LTS

Step 2) Once the installation complete, you can check the Python 2 version using the “–version” command.

python2 --version
python3 --version

Check the python version installed
Check the python version installed

From the image above, we can see we are running Python 2.7 and Python 3.8. Now, let’s jump on to one of the important parts of this tutorial of how to configure Python and switch between the two versions – Python 2 and Python 3.

How to switch between Python 2 and 3 versions on Ubuntu 20.04

Method 1: My recommended way by configuring

Step 1) Check all the available Python versions in your system. To do so, we will need to check the /bin directory. That is because we can have variations in Python 3. Say Python 3.7 and Python 3.8. In such situations, the –version command won’t be useful as it only lists the currently configured version.

Execute the commands below on the Terminal.

ls /usr/bin/python*

Check all the installed Python versions in the bin directory
Check all the installed Python versions in the bin directory

Step 2) Once we have listed all the versions present on the system, we need to check whether there are any Python-alternatives configured.

Execute the command below on the Terminal.

sudo update-alternatives --list python

Check for any Python alternatives configured on the system
Check for any Python alternatives configured on the system

From the image above, we see that there are no Python alternatives configured.

Step 3) Now, we will configure two Python alternatives. From the image in Step 2 above, we saw that I have Python 2.7 and Python 3.8 present on my system.

Execute the commands below on the Terminal.

$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 1
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 2

Configure Python Alternatives
Configure Python Alternatives

Step 4) We need to confirm the Python alternatives set and whether they are in use.
Execute the command below again.

$ sudo update-alternatives --config python

Confirm the Python Alternatives set
Confirm the Python Alternatives set

On the prompt that appears on the Terminal, enter 1 or 2 to make your selection. In this post, we want to use Python 2; therefore, we will enter option 1.

Step 5) Now, let’s check the Python version currently running on our system. Execute the command below.

python --version

Check the currently running Python Version
Check the currently running Python Version

To switch to another Python version, all you need to do is execute the command on Step 4 and select the other option. In this case, we would choose option 2 to use Python 3.

Method 2: Old-school way without configuration

This other method, we would refer to it as the manual way since we won’t perform any configurations. Follow the procedure below.

Step 1) Install Python 2 with the command below.

sudo apt install python2

Step 2) Check the Python versions present in your system by running the command below.

ls /usr/bin/python*

Check all the installed Python versions in the bin directory
Check all the installed Python versions in the bin directory

Step 3) In this post, we currently have Python 2 and Python 3 available. Now write your Python 2 or Python 3 code. 

For example, below is a Python 2 code to print the sentence “Hello, This Fosslinux.com.

Python2 Code
Python2 Code

Step 4) To execute your code using Python 2, you will need to specify the version manually. For example, the python file is called Example.py. Execute the command below.

python2 Example.py

Execute the Python 2 code
Execute the Python 2 code

If we used Python 3 to execute the code below, we would get an error, as shown below.

Error when using Python3 to execute Python2 code
Error when using Python3 to execute Python2 code

If we were writing code using Python 3, we would specify Python 3 when running the program.

Conclusion

Despite Python 2 being dropped in the recent Linux releases, some people still find it advantageous and use it in their development. There are vast applications built using Python 2, and the transition to Python 3 has not been put into effect fully.

These and many other reasons make Python 2 still relevant in the development community. With the methods described above, you will be able to switch from one Python version to another easily when coding.

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