Installing Python3.8 on Ubuntu 18.04
By default Ubuntu 18.04 includes Python 3.6. If you want a later version of Python (namely 3.7, 3.8 or 3.9), it is possible to install them using an apt PPA. PPAs, or ‘Personal Package Archives’, are additional repositiories that apt can use to search for packages, but they can contain packages that haven’t undergone enough testing and validation to end up in one of the main repos: this means that they may not receive timely updates, they might have security problems, they might be seriously broken, etc. Consider this a caveat emptor.
The PPA you’ll need to install a later version of Python 3 is the deadsnakes PPA; deadsnakes has a copy of each version of Python from 2.3 through 3.9, so you can grab earlier versions as well.
To begin, add the PPA and run update to grab the package list.
sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt update
Then just install the version of Python you need:
sudo apt install python3.7
sudo apt install python3.8
sudo apt install python3.9
Note that it won’t be installed over your system copy of Python 3, so you’ll need to invoke it with python3.X rather than python or python3.