virtualenv is a tool to create isolated Python environments. 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).

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鈥檚 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鈥檛 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鈥檛 share libraries with other virtualenv environments (and optionally doesn鈥檛 access the globally installed libraries either).

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

Bernat Gabor: status quo of virtual environments

Carl Meyer: Reverse-engineering Ian Bicking鈥檚 brain: inside pip and virtualenv