How to use Virtualenv in Python

This post will describe what Virtualenv is and how you can use it.

What is Virtualenv?

Virtualenv is a tool to create isolated Python environments, it's perhaps the easiest way to configure a custom Python environment.

Virtualenv allows you to add and modify Python modules without access to the global installation.

What does it do?

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/python2.7/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

How to install Virtualenv?

There are a few ways to install virtualenv on your machine.

You can use either the source tarball, pip or by using easy_install.
$ sudo easy_install virtualenv

Searching for virtualenv
Best match: virtualenv 1.8.2
processing virtualenv-1.8.2.tar.gz
Processing dependencies for virtualenv
Finished processing dependencies for virtualenv
Source ball installation
Get the latest version from here:
tar xzvf virtualenv-1.8.tar.gz
python virtualenv-1.8/ $HOME/env
Pip installation
pip install virtualenv


To create virtual environments, you can use the virtualenv command.

Create an environment called "foobar":
virtualenv foobar

Activate the environment by sourcing its activate script, which is located in the environment's bin/ directory:
source foobar/bin/activate

This will change your $PATH so its first entry is the virtualenv’s bin/ directory.

If you install a package in your virtual environment, you'll see that executable scripts are placed in foobar/bin/ and eggs in foobar/lib/python2.X/site-packages/easy_install yolk

Yolk is a small command line tool which can, among other things, list the currently installed Python packages on your system:
yolk -l

Virtualenv inherits packages from the system's default site-packages directory.

This is especially useful when relying on certain packages being available, so you don't have to go through installing them in every environment.

To leave an environment, simply run deactivate:

If you execute he yolk command now, you will see that it won't work because the package was installed only in your virtual environment.

Once you reactivate your environment it will be available again.
I used different sources to find information for this article: 
the official virtualenv website, from Chris Scott "A Primer on virtualenv" and from Arthur Koziel 'Working with virtualenv'

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