Python Virtual Environments

Environment Isolation Requirements

Setting up the correct virtual environment depends on the degree of isolation one requires from their system.

If one doesn't require any isolation (i.e. the system is not dependent on specific Python or package versions), then continue using the system's standard package manager to manage package versions, for example, pacman on Arch Linux or pip on other operating systems; and then reinstall the required Python version as necessary.

However, if isolation is required, follow one of the below options based on one's needs:

• If different package versions are required, but not different Python versions, skip to the Using the venv Module section.
• If both different package and Python versions are required, continue reading.

Acquiring Specific Python Versions

The most reliable method of acquiring a specific Python version is to compile it from source. See the Compiling Python from Source Code guide for more information.

Using the venv Module

Ensure there exists a valid Python installation for the specific Python version one intends to use (i.e. a working Python executable).

Note that venv is only available from Python 3 onwards.

Creating Virtual Environments

The general syntax to create a virtual environment is:

<python-executable> -m venv <venv-directory>

• Replace <python-executable> with python to use the system's Python version, or, replace it with the path to the desired Python executable.
• Replace <venv-directory> with the path to where the virtual environment is to be created.

For example, say one has multiple Python versions installed to ~/dev/pyexes, and keeps all their virtual environments in ~/dev/venvs, then the following command can be used to create a Python 3.9 virtual environment called venv-py39:

dev/pyexes/python39/bin/python3.9 -m venv dev/venvs/venv-py39

After executing the above command, the venv-py39 directory will contain a few subdirectories: the important one is the bin subdirectory which will contain a copy of the Python executable along with the utilities for the virtual environment. As a result, each baseline virtual environment will consume approximately 20 MB.

Activating Virtual Environments

The general syntax to activate a virtual environment is:

source <path-to-venv>/bin/activate

For example (continuing with the prior scenario), to activate the virtual environment called venv-py39 in ~dev/venvs:

source dev/venvs/venv-py39/bin/activate

Note that after activating the virtual environment, the command line will be prefixed with the name of the virtual environment, e.g. (venv-py39). This prefix has been added to the following commands to indicate that these commands have to be executed within the virtual environment.

To confirm that the virtual environment and installation is all working correctly, run the following commands and confirm the output is as expected:

(venv-py39) which python
# /home/<username>/dev/venvs/venv-py39/bin/python

(venv-py39) which pip   
# /home/<username>/dev/venvs/venv-py39/bin/pip

(venv-py39) python --version
# Python 3.9.16

Installing Packages Inside the Virtual Environment

Install a package by name using the pip package manager:

pip install <package-name>

Install a package by name and version using the pip package manager:

pip install <package-name>==<version>

Install all packages listed in a requirements plain text file:

pip install -r <path/to/requirements.txt>

Deactivating and Deleting Virtual Environments

To deactivate the virtual environment:

deactivate

To delete the virtual environment (first ensure it is deactivated):

rm -rf ~/dev/venvs/venv-py39