I've never had this kind of problem because I'm using PyCharm and it has perfect version control.
But now I'm using flask and running it in terminal using 'flask run' command and I've just noticed that it runs on python 3.5:
.local/lib/python3.5/site-packages/
When I type python --version it says '3.6.5' but I went to this ~/.local/lib directory and noticed that there's only 2 folders: python2.7 and python3.5
When I try to update python using
sudo apt-get install python3.6
It says 'Requirement already satisfied.'
The reason I need python3.6 here because I can't run a flask app that contains an f-string.