Ubuntu 20.04

https://www.liquidweb.com/kb/how-to-install-and-update-python-to-3-9-in-ubuntu

# optional installation steps
fab@fvm:~$ sudo apt-get update
fab@fvm:~$ sudo apt-get install software-properties-common
fab@fvm:~$ sudo add-apt-repository ppa:deadsnakes/ppa
fab@fvm:~$ sudo apt-get update
fab@fvm:~$ sudo apt-get install python3.9 python3.9-venv python3.9-dev

fab@fvm:~/srv$ mkdir my-project  # step 1
fab@fvm:~/srv$ cd my-project

fab@fvm:~/srv/my-project$ which python3.9  # check system python
/usr/bin/python3.9  # <-- system python3.9

fab@fvm:~/srv/my-project$ python3.9 -m venv lpp3ed  # step 2
fab@fvm:~/srv/my-project$ source ./lpp3ed/bin/activate  # step 3

# check python again: now using the virtual environment's one
(lpp3ed) fab@fvm:~/srv/my-project$ which python
/home/fab/srv/my-project/lpp3ed/bin/python

(lpp3ed) fab@fvm:~/srv/my-project$ python  # step 4
Python 3.9.2 (default, Feb 20 2021, 20:56:08)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()

(lpp3ed) fab@fvm:~/srv/my-project$ deactivate  # step 5
fab@fvm:~/srv/my-project$


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Windows 10
Simply install from python website, then from terminal:

C:\Users\Fab\srv>mkdir my-project  # step 1

C:\Users\Fab\srv>cd my-project

C:\Users\Fab\srv\my-project>where python  # check system python
C:\Users\Fab\AppData\Local\Programs\Python\Python39\python.exe
C:\Users\Fab\AppData\Local\Microsoft\WindowsApps\python.exe

C:\Users\Fab\srv\my-project>python -m venv lpp3ed  # step 2

C:\Users\Fab\srv\my-project>lpp3ed\Scripts\activate  # step 3

# check python again, now virtual env python is listed first
(lpp3ed) C:\Users\Fab\srv\my-project>where python
C:\Users\Fab\srv\my-project\lpp3ed\Scripts\python.exe
C:\Users\Fab\AppData\Local\Programs\Python\Python39\python.exe
C:\Users\Fab\AppData\Local\Microsoft\WindowsApps\python.exe

(lpp3ed) C:\Users\Fab\srv\my-project>python  # step 4
Python 3.9.2 (tags/v3.9.2:1a79785, Feb 19 2021, 13:44:55)
⇢ [MSC v.1928 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> exit()

(lpp3ed) C:\Users\Fab\srv\my-project>deactivate  # step 5
C:\Users\Fab\srv\my-project>