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 "", line 1, in 
ImportError: No module named django
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