Virtual Environments¶

A virtual environment is a local Python environment isolated from the system-wide environment.

virtualenv¶

The term “virtualenv” can refer to the command virtualenv, used to create a virtual environment, or to the virutal environment itself.

virtualenvwrapper¶

virtualenvwrapper is a set of extensions to the virtualenv tool. The extensions include wrappers for creating and deleting virtual environments and otherwise managing your development workflow, making it easier to work on more than one project at a time without introducing conflicts in their dependencies [1].

$ mkvirtualenv class
New python executable in class/bin/python
Installing distribute.............................................................................................................................................................................................done.
Installing pip...............done.
virtualenvwrapper.user_scripts creating /home/jason/.virtualenvs/class/bin/predeactivate
virtualenvwrapper.user_scripts creating /home/jason/.virtualenvs/class/bin/postdeactivate
virtualenvwrapper.user_scripts creating /home/jason/.virtualenvs/class/bin/preactivate
virtualenvwrapper.user_scripts creating /home/jason/.virtualenvs/class/bin/postactivate
virtualenvwrapper.user_scripts creating /home/jason/.virtualenvs/class/bin/get_env_details
(class)$ pip freeze # A few packages are installed by default
argparse==1.2.1
distribute==0.6.24
wsgiref==0.1.2

Note that when the virtual environment is active, its name (in this case “class”) is prepended to the shell prompt:

$ # Ordinary shell prompt
(class)$ # Virtual environment "class" is active

If later you have logged out, and want to activate this virtual environment, you can use the workon command:

$ workon class
(class)$

You can deactivate the virtual environment with the deactivate command:

(class)$ deactivate
$ # Back to normal shell prompt

Location of Virtualenvs¶

By default, virtualenvwrapper stores your virtual environments in ~/.virtualenvs. However you can control this by setting the WORKON_HOME environment variable. This could potentially be used for shared virtual environments, perhaps with group write permission.

export WORKON_HOME=/path/to/virtualenvs

Virtual Environments for Scripts¶

There are several ways you can run scripts that rely on a virtualenv:

Use Fabric’s prefix() context manager when calling the script remotely:

def task():
    with prefix('workon class'):
        run('uptime')
        run('uname -a')

Have whatever is calling your script (cron etc) call workon first.
Specify your virtualenv’s Python interpreter directly in the script’s bangline.
Use a bash script as a wrapper. Ugly, but sometimes convenient.

Footnotes

[1]	http://www.doughellmann.com/projects/virtualenvwrapper/