Have you ever upgraded one python package for one project and then find out it breaks the package dependencies of other projects? This is where virtualenv shines. As the name indicates, it creates a ‘virtual’ environment for each project as if its dependences start from a clean installation of python.

If you have pip, installing virtualenv is simply

pip install virtualenv


To create a virtual environment, use

virtualenv venv


I use python2 by default, the shell command above creates a folder called venv. Of course you can call this folder whatever, but venv is somewhat popular. It is also common to place this venv folder inside the project folder.

To create a virtual environment with python3, use

virtualenv -p /usr/bin/python3 python3


To activate a virtual environment, use

source venv/bin/activate


You can tell the virtual environment is activated by looking at the shell prompt. For example, on my computer the prompt changes from nos ~ $ to (venv) nos ~$ where nos is my user name.

After activating the virtual environment, you can install packages using pip as usual.

To deactivate a virtual environment, simply type

deactivate


and the command line prompt changes accordingly.

To check the packages installed for the virtual environment, use

pip freeze