In this config we will setup a python virtualenv. The intention
behind the use of virtualenv as a python development environment is
to separate global system installed executables and libraries with packages
installed directly inside virtualenv. As a result, one is able to
test a given python application with multiple system environments on a single
host.

The installation of virtualenv on Debian Linux is simple as:

# apt-get install python-virtualenv

At this stage we are able to create our fisrt virtualenv python
development environment eg. myapp

$ virtualenv --no-site-packages myapp
Running virtualenv with interpreter /usr/bin/python2
New python executable in myapp/bin/python2
Also creating executable in myapp/bin/python
Installing setuptools, pip...done.

By using the --no-site-packages option we ensure that no system
global packages will be linked to our new virtualenv environment.
Let’s explore in more detail what just happened. Firstly,
virtualenv has created a new directory to act as a container for
our myapp development.

$ tree -L 2 myapp/
myapp/
├── bin
│   ├── activate
│   ├── activate.csh
│   ├── activate.fish
│   ├── activate_this.py
│   ├── easy_install
│   ├── easy_install-2.7
│   ├── pip
│   ├── pip2
│   ├── pip2.7
│   ├── python -> python2
│   ├── python2
│   └── python2.7 -> python2
├── lib
│   ├── python2.7
│   └── python-wheels
└── local
 ├── bin -> /home/debian/myapp/bin
 └── lib -> /home/debian/myapp/lib

7 directories, 12 files

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As you can see our new virtualenv environment contains all
necessary tools to shape our environment such as separate python, pip and
easy_install binaries.

At this stage we are ready to enter virtualenv environment:

$ cd myapp/
$ source bin/activate
(myapp)debian@hostname:~/myapp$

One you enter virtualenv python environment your PATH to
binary executables will change:

$ echo $PATH
/home/debian/myapp/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

As a result all your python commands will be executed using by using
virtualenv environment binaries. Use which command to locate your
a full path to your python binaries:

(myapp)debian@hostname:~/myapp$ which python pip easy_install
/home/debian/myapp/bin/python
/home/debian/myapp/bin/pip
/home/debian/myapp/bin/easy_install

Now, we are ready to install packages without affecting the global system wide
environment. As an example we can install django package local to
only our myapp environment:

(myapp)debian@hostname:~/myapp$ pip install django
Downloading/unpacking django
 Downloading Django-1.8-py2.py3-none-any.whl (6.2MB): 6.2MB downloaded
Installing collected packages: django
Successfully installed django
Cleaning up...

Test django package installation:

(myapp)debian@hostname:~/myapp$ python -c "import django; 
print(django.get_version())"
1.8

All is working as expected. Once you finish your work with virtualenv
environment you can exit by using deactivate function provided
within bin/activate script:

(myapp)debian@hostname:~/myapp$ deactivate 
debian@hostname:~/myapp$ 

An attempt to access django packages installed within
virtualenv environment will now fail:

$ python -c "import django; print(django.get_version())"
Traceback (most recent call last):
 File "