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Problem: Arch Linux recently updated Python to 3.9, but some projects only work with Python <= 3.8.x.

Solution: Use pyenv to install other Python versions.

Setup

Install pyenv:

# pacman -S pyenv

Installing

List possible Python versions:

$ pyenv install --list | grep -E ' [2-3]{1}.[0-9]+.[0-9]+'

Install a specific Python version (e.g. 3.6.12):

$ pyenv install -v 3.6.12

See installed versions:

$ ls ~/.pyenv/versions

or:

$ pyenv versions

Uninstalling

Removing a Python version:

$ rm -r ~/.pyenv/versions/3.6.12

or:

$ pyenv uninstall 3.6.12

Switching

See currently active version of python (initially, same as which python):

$ pyenv which python
/usr/bin/python

Change python version globally:

$ pyenv global 3.6.12

$ which python
/usr/bin/python

$ pyenv which python
~/.pyenv/versions/3.6.12/bin/python

Revert to default version:

$ pyenv global system

The Python version can be switched on different levels, using the respective commands and way to store the version number:

  1. system: manage using your OS package manager or manually (outside of pyenv)
  2. global: pyenv global <version>
    • ~/.pyenv/version
  3. local: pyenv local <version>
    • .python-version
  4. shell: pyenv shell <version>
    • $PYENV_VERSION

Initialize the version:

$ eval "$(pyenv init -)"

Virtual Environment

Install the pyenv-virtualenv plugin:

$ git clone https://github.com/pyenv/pyenv-virtualenv.git \ 
    "$(pyenv root)/plugins/pyenv-virtualenv"

Activate and init the desired Python version:

$ pyenv local 3.6.12
$ eval "$(pyenv init -)"

Create a virtual environment called myproject and activate it:

$ pyenv virtualenv 3.6.12 myproject
$ pyenv local myproject
$ pyenv which python
~/.pyenv/versions/myproject/bin/python

$ eval "$(pyenv virtualenv-init -)"

Reactivate the next time:

$ eval "$(pyenv init -)"
$ eval "$(pyenv virtualenv-init -)"

Add those commands to the ~/.bashrc, or, to save some ~0.5 seconds when opening a shell, create a function to be invoked if pyenv is needed in the respective shell session:

function p {
    eval "$(pyenv init -)"
    eval "$(pyenv virtualenv-init -)"
}

Then activate everything as follows:

$ p

Example: Additional Project

After the initial setup is done, and a Python version (e.g. 3.8.6) installed, a new project can be configured with a virtual environment to use that version.

First, move to the project folder:

$ cd ~/myproject

Second, use the installed python version for that particular project:

$ pyenv local 3.8.6

Third, install a virtual environment:

$ pyenv virtualenv myproject

Fourth, activate the virtual environment:

$ pyenv local myproject

Fifth, activate the environment using the p function:

$ p