Virtualenv is really a nice thing if you want to work with different python version.

suppose you have a project that need python 2.7 and in the mean time you also want to work on another project that need python 3.5. What can you do then? What is the solution for you?

Install virtualenv

sudo pip install virtualenv

Create a virtualenv for project

cd project_folder
virtualenv env_name

Activate virtualenv

‘env_name’ virtualenv is create and you can use this now.
source env_name/bin/activate

Install packages as you want in your virtualenv.

For deactivate this write