How to use Python virtualenv

What is Virtualenv?

A Virtual Environment, put simply, is an isolated working copy of Python which
allows you to work on a specific project without worry of affecting other projects

It enables multiple side-by-side installations of Python, one for each project.

It doesn’t actually install separate copies of Python, but it does provide a
clever way to keep different project environments isolated.

Verify if Virtualenv is installed

There is a chance that virtualenv is already installed on your system. 

Run the following command in your terminal

virtualenv --version

If you see a version number (in my case 1.6.1), it's already installed.
>>1.6.1

Install Virtualenv

There are a number of ways to install virtualenv on your system.

$ sudo apt-get install python-virtualenv

$ sudo easy_install virtualenv

$ sudo pip install virtualenv

Setup and Use Virtualenv

Once you have virtualenv installed, just fire up a shell and create your own
environment.

First create a directory for your new shiny isolated environment

mkdir ~/virtualenvironment

To create a folder for your new app that includes a clean copy of Python,
simply run:

virtualenv ~/virtualenvironment/my_new_app

(add --no-site-packages if you want to isolate your environment from the main site
packages directory)

To begin working with your project, you have to cd into your directory (project)
and activate the virtual environment.

cd ~/virtualenvironment/my_new_app/bin

Lastly, activate your environment:

source activate

Notice how the prompt of your shell changed to show the active environment. 

That is how you can see that you're in your new environment.

Any packages you install now using pip or easy_install get installed into
my_new_app/lib/python2.7/site-packages.

To exit your virtualenv just type “deactivate”.

What did Virtualenv do?

Packages installed here will not affect the global Python installation. 

Virtualenv does not create every file needed to get a whole new python environment

It uses links to global environment files instead in order to save disk space end
speed up your virtualenv. 

Therefore, there must already have an active python environment installed on your
system.

Install a package in your Virtualenv

If you look at the bin directory in your virtualenv, you’ll see easy_install which
has been modified to put eggs and packages in the virtualenv’s site-packages 
directory.

To install an app in your Virtualenv:

pip install flask

You don't have to use sudo since the files will all be installed in the virtualenv
/lib/python2.7/site-packages directory which was created as your own user account

That's it, I hope that you learned something from this post

For further reading, please see:
http://flask.pocoo.org/docs/installation/#virtualenv
http://pypi.python.org/pypi/virtualenv
Getting-started-with-virtualenv-isolated-python-environments/

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