virtualenv¶

virtualenv is a tool to create isolated Python environments.

virtualenv vs venv¶

Since Python 3.3, a subset of it has been integrated into the standard library under the venv module. The venv module does not offer all features of this library, to name just a few more prominent:

is slower (by not having the app-data seed method),
is not as extendable,
cannot create virtual environments for arbitrarily installed python versions (and automatically discover these),
is not upgrade-able via pip,
does not have as rich programmatic API (describe virtual environments without creating them).

Concept and purpose of virtualenv¶

The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these libraries? If you install everything into your host python (e.g. python3.8) it’s easy to end up in a situation where two packages have conflicting requirements.

Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application. Also, what if you can’t install packages into the global site-packages directory, due to not having permissions to change the host python environment?

In all these cases, virtualenv can help you. It creates an environment that has its own installation directories, that doesn’t share libraries with other virtualenv environments (and optionally doesn’t access the globally installed libraries either).

Compatibility¶

With the release of virtualenv 20.22, April 2023, (release note) target interpreters are now limited to Python v. 3.7+.

Trying to use an earlier version will normally result in the target interpreter raising a syntax error. This virtualenv tool will then print some details about the exception and abort, ie no explicit warning about trying to use an outdated/incompatible version. It may look like this:

$ virtualenv --discovery pyenv -p python3.6 foo
RuntimeError: failed to query /home/velle/.pyenv/versions/3.6.15/bin/python3.6 with code 1 err: '  File "/home/velle/.virtualenvs/toxrunner/lib/python3.12/site-packages/virtualenv/discovery/py_info.py", line 7
    from __future__ import annotations
    ^
SyntaxError: future feature annotations is not defined

In tox, even if the interpreter is installed and available, the message is (somewhat misleading):

py36: skipped because could not find python interpreter with spec(s): py36

Useful links¶

Related projects, that build abstractions on top of virtualenv

virtualenvwrapper - a useful set of scripts for creating and deleting virtual environments
pew - provides a set of commands to manage multiple virtual environments
tox - a generic virtualenv management and test automation command line tool, driven by a tox.ini configuration file
nox - a tool that automates testing in multiple Python environments, similar to tox, driven by a noxfile.py configuration file

Tutorials

Corey Schafer tutorial on how to use it
Using virtualenv with mod_wsgi

Presenting how the package works from within