Python virtual environments¶

In order to ease the management of the project dependencies, it is recommended to make use of Python virtual environments. Virtual environments are used to provide isolation between projects, keeping all the required libraries in a directory available only to the virtual environment, and thus avoid polluting the global sites-packages directory.

There is a utility called virtualenvwrapper that simplifies the management of virtual environments, and we will use this utility to obtain a clean virtual environment for our project.

To install this utility, run the following command (root permissions may be necessary):

$ pip install virtualenvwrapper

Once the installation has finished, open up the .bashrc file and add the following lines:

export WORKON_HOME=$HOME/.virtualenvs      # Virtual environments folder
source /usr/local/bin/virtualenvwrapper.sh # Enable virtualenvwrapper commands

To create a new virtual environment, run the following command:

$ mkvirtualenv uvispace # Syntax: mkvirtualenv (name for the virtual environment)
(uvispace) $            # Prompt gets updated to reflect the virtual environment is active

In case the virtual environment is already created, it is only necessary to activate it with the following command:

$ workon uvispace # Syntax: workon (name for the virtual environment)
(uvispace) $      # Prompt gets updated to reflect the virtual environment is active

If we want to stop using a virtual environment, run the following command:

(uvispace) $ deactivate
$                        # Prompt gets updated to reflect the virtual environment is no longer active

By default, mkvirtualenv uses the system default Python interpreter. If a specific version of python is required, it can be specified with the -p parameter:

$ mkvirtualenv -p python2 (name)    # Python2 virtual environment
$ mkvirtualenv -p python3.4 (name)  # Python3.4 virtual environment

When entering a new virtual environment, there will be no access to the software libraries previously installed on the system, as the purpose of this tool is to have an isolated environment. To add a new library to the virtual environment, you only have to install it using pip (remember to previously activate the environment), and the same rule works for other pip commands like uninstall or freeze:

(uvispace) $ pip install <library-name>

Finally, certain projects, like uvispace, have predefined libraries and versions and they are specified under a requirements.txt file (The name does not have to be necessary the same). In this case, these libraries can be automatically installed on the virtual environment using the following command:

(uvispace) $ pip install -r requirements.txt

Other useful commands are the following:

$ lsvirtualenv        # List all available virtual environments
$ rmvirtualenv (name) # Deletes a virtual environment

A more extensive reference on the virtualenvwrapper utility can be found on its documentation.