virtualenv is a software to create isolated Python environments.
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 applications? If you install everything into /usr/lib/python2.4/site-packages (or whatever your platform's standard location is), it's easy to end up in a situation where you unintentionally upgrade an application that shouldn't be upgraded.
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? For instance, on a shared host.
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 use the globally installed libraries either).
The basic usage is:
$ python virtualenv.py ENV
This creates ENV/lib/python2.4/site-packages (or ENV/lib/python2.5/site-packages on Python 2.5, etc), where any libraries you install will go. It also creates ENV/bin/python, which is a Python interpreter that uses this environment. Anytime you use that interpreter (including when a script has #!/path/to/ENV/bin/python in it) the libraries in that environment will be used. (Note for Windows: scripts and executables on Windows go in ENVScripts; everywhere you see bin/ replace it with Scripts)
It also installs Setuptools for you, and if you use ENV/bin/easy_install the packages will be installed into the environment.
What's New in This Release: [ read full changelog ]
· Updated to pip 1.3.1 that fixed a major backward incompatible change of parsing URLs to externally hosted packages that got accidentily included in pip 1.3.