Python Virtual Environment

Python virtual environments are used to keep a version of the python interpreter, installed packages, and other binaries (such as pip) in a separate work environment. This is very useful when switching versions of python or avoiding conflicting packages/versions. There are software packages available to help manage virtual environments, such as Anaconda (or Conda). Conda is now charging a subscription for commercial use and will block access from corporations unless a paid subscription is used. These instructions show how to manage python virtual environments using the built-in features of Python with a little help from bash/zsh.

Instructions for MacOS

Although MacOS comes with Python (as of Big Sur: both 2.7 and 3.7) which is used by the OS. It's best practice to use virtual environments rather than install packages or modify the interpreters used by the OS.

Choose A Folder

Each virtual environment, keeps symlinks and files in a directory structure. It's a good idea to decide at the start where to put the virtual environment directories. They can be stored anywhere on the filesystem. One option is to use a common directory. For the purposes of these instructions, we will use a commmon directory called envs under the home directory. Create the folder with the following command:

• mkdir ~/envs

Install Python

Install the desired version of Python by going to python.org. Download and install one or more versions of python. The installer package will install each one into a separate directory on the system to support multiple versions. At the time of this writing they are installed into /Library/Frameworks/Python.framework/Versions/X.X where X.X is the version. This path may be needed when creating a virtual environment through an IDE.

Install The Virtual Environment Package

This only needs to be done once for each version of Python that will be used in virtual environments. Use the command below, replacing X.X with the version installed above.

• sudo pipX.X install virtualenv

Create a Virtual Environment

Create a virtual environment by running pythonX.X -m virtualenv path-to-env Here's an example using Python 3.9 and creating a virtual environment called test_39

• python3.9 -m virtualenv ~/envs/test_39

Activate a Virtual Environment

The active environment will dictate where pip installs packages and which python interpreter and other binaries will run. By default, the shell prompt will have the name of the currently active environment in parentheses. Activate the environment by sourcing the respective activate file into the current shell replacing name with the name of the desired environment:

• source ~/envs/name/bin/activate

Default Virtual Environement

Place the source command for the desired environment in ~/.zshrc for zsh or ~/.bashrc for bash. Older MacOS (and those user accounts upgraded from older OS) will have your user set to bash. Newer accounts default to zsh. Check your user settings if you are unsure.

Deactivate Virtual Environment

To revert back to the sysetem default python, enter the command: deactiavate

Deleting Virtual Environment

Simply delete the directory with the name of the virtual environment you wish to delete. It's advised to use Finder to delete the folder rather than shell commands. This will help reduce undesired results if a command is typed in wrong.

Command Shortcut For Activating Virtual Environements

The source command (above) used to activate a virtual environment is a bit cumbersome. One way to improve this, is to place a function in the shell rc file that will list and change the environment. Place the below lines in ~/.zshrc (or ~/.bashrc if using bash). Restart the terminal session or source the rc file for it to take affect. Enter e to list the environment names and e name to activate it (replacing name with the environement name).

e() {
  if [ -z "$1" ]
  then
    ls -1 ~/envs
  else
    source ~/envs/$1/bin/activate
  fi
}