Virtualenv

Virtualenv is a tool that lets you create an isolated Python environment for your project. It creates an environment that has its own installation directories, that doesn’t share dependencies with other virtualenv environments (and optionally doesn’t access the globally installed dependencies either). You can even configure what version of Python you want to use for each individual environment. It's very much recommended to use virtualenv when dealing with Python applications.

Installation

To install virtualenv run:

$ pip install virtualenv

Usage

If you have a project in a directory called my-project you can set up virtualenv for that project by running:

$ cd my-project/
$ virtualenv venv

If you want your virtualenv to also inherit globally installed packages run:

$ virtualenv venv --system-site-packages

These commands create a venv/ directory in your project where all dependencies are installed. You need to activate it first though (in every terminal instance where you are working on your project):

$ source venv/bin/activate

You should see a (venv) appear at the beginning of your terminal prompt indicating that you are working inside the virtualenv. Now when you install something like this:

$ pip install <package>

It will get installed in the venv/ folder, and not conflict with other projects.

To leave the virtual environment run:

$ deactivate

Important: Remember to add venv to your project's .gitignore file so you don't include all of that in your source code.

It is preferable to install big packages (like Numpy), or packages you always use (like IPython) globally. All the rest can be installed in a virtualenv.

Virtualenvwrapper

To make it easier to work on multiple projects that has separate environments you can install virtualenvwrapper. It's an extension to virtualenv and makes it easier to create and delete virtual environments without creating dependency conflicts.

To install virtualenvwrapper run:

$ pip install virtualenvwrapper

Depending on your setup you might need to install it using sudo. Read the installation documentation for more information.

Note: virtualenvwrapper keeps all the virtual environments in ~/.virtualenv while virtualenv keeps them in the project directory.