Managing Python Versions with Pyenv

The pyenv command line utility is convenient tool for managing python versions.

Installation

It’s best to follow the above link for general installation instructions. Otherwise, I will assume you are using macOS with Fish Shell.

First install pyenv and pyenv-virtualenv with brew:

brew install pyenv pyenv-virtualenv

You also want to add some lines to your config.fish:

set -Ux PYENV_ROOT "$HOME"/.pyenv
status is-login; and pyenv init --path | source
status is-interactive; and pyenv init - | source
status is-interactive; and pyenv virtualenv-init - | source
alias brew="env PATH=(string replace (pyenv root)/shims '' \"\$PATH\") brew"

You can set PYENV_ROOT to be any location you would like. The remaining commands are used to initialize pyenv and add the corresponding python versions to your PATH.

Startup

The first thing to do is to set your preferred global python version:

pyenv global 2.7.18 3.10.1

sets the python2 version to 2.7.18 and the python3 version to 3.10.1. The global version is useful for user-wide modules and tools you might want to install with pip. You can get a list of possible versions with pyenv install -l.

Managing virtual environments

For specific projects, you probably only want to install the packages necessary for that project. A virtualenv is essentially an isolated installation of python (and packages) which are specific to the current environment. This lets you organize what modules you have installed, as well as the python version.

As an example, let’s create a virtualenv named my-venv. First, create it with

pyenv virtualenv 3.10.1 my-venv

Let’s say we are in a project directory where I want to use the my-venv virtual environment. Simply create a file named .python-version, which is populated with the name of the desired virtual environment:

echo "my-venv" > .python-version

Now, whenever you enter this directory, pyenv will automatically activate the environment, and whenever you leave, the environment will deactivate.

You can also manually activate the virtual environment with

pyenv activate my-venv

and deactivate with

pyenv deactivate

To get a list of all the virtual environments currently installed, run

pyenv virtualenvs

You can uninstall virtual environments with

pyenv uninstall my-venv