How to downgrade Python 3.6.9 to 3.4.x on Ubuntu 18.04?

Python has this neat feature called virtual environments where you can setup different python versions for different projects. The documentation is pretty straightforward.

I would also recommend reading similar discussion that already exist.

As a final note I always name my virtual environment venv as its both simple and readable.

I highly recommend using Pyenv to manage versions. You won't have to downgrade anything, just choose a different global python version.

Ubuntu relies on Python 3.6. If you remove/change the system Python install, parts of the OS will break, like the terminal. Use a virtualenv instead.

First, you can use the deadsnakes PPA to install Python 3.4:

sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt-get update
sudo apt-get install python3.4 python3.4-venv

Then set up the virtualenv. See the tutorial and venv documentation. The crucial command will be something like this:

python3.4 -m venv ...