While installing Python and Virtualenv on Mac OS X 10.9 Mavericks can be done several ways, this tutorial will guide you through the process of configuring a stock Mavericks installation into a solid Python development environment.

First steps

This guide assumes that you have already installed Xcode (or at least the command-line developer tools) and Homebrew. For details, please follow the steps in the Mavericks Configuration Guide.

Python

We are going to install the latest 2.7.x version of Python via Homebrew. Why bother, you ask, when Apple includes Python along with Mavericks? Here are some reasons:

• When using the bundled Python upgrading OS X can nuke your Python packages, forcing you to reinstall them.
• As new versions of Python are released, the Python bundled with OS X will become out-of-date. Homebrew always has the most recent version.
• Apple has made significant changes to its bundled Python, potentially resulting in hidden bugs.
• Homebrew’s Python includes the latest Python package management tools: pip and Setuptools

Along the same lines, the version of OpenSSL that comes with Mavericks is out-of-date, so we’re going to tell Homebrew to download the latest OpenSSL and compile Python with it.

Use the following command to install Python via Homebrew:

brew install python

You’ve already modified your PATH as mentioned in the Mavericks Configuration Guide, right? If not, please do so now.

Optionally, we can also install Python 3.x alongside Python 2.x:

brew install python3

… which makes it easy to test your code on both Python 2.x and Python 3.x.

Pip

Let’s say you want to install a Python package, such as the fantastic virtualenv environment isolation tool. While nearly every Python-related article for Mac OS X tells the reader to install it via sudo easy_install virtualenv, the downsides of this method include:

1. installs with root permissions
2. installs into the system /Library
3. pip is more actively developed than Setuptools’ easy_install
4. using the tools provided by Homebrew yields a more reliable Python environment

As you might have guessed by now, we’re going to use the tools provided by Homebrew to install the Python packages that we want to be globally available. When installing via Homebrew Python’s pip, packages will be installed to /usr/local/lib/python2.7/site-packages, with binaries placed in /usr/local/bin.

Version control (optional)

The first thing I pip-install is Mercurial, since I have Mercurial repositories that I push to both Bitbucket and GitHub. If you don’t want to install Mercurial, you can skip ahead to the next section.

The following command will install Mercurial and hg-git:

pip install Mercurial hg-git

At a minimum, you’ll need to add a few lines to your .hgrc file in order to use Mercurial:

vim ~/.hgrc

The following lines should get you started; just be sure to change the values to your name and email address, respectively:

[ui]
username = YOUR NAME <address@example.com>

To test whether Mercurial is configured and ready for use, run the following command:

hg debuginstall

If the last line in the response is “no problems detected”, then Mercurial has been installed and configured properly.

Virtualenv

Python packages installed via the steps above are global in the sense that they are available across all of your projects. That can be convenient at times, but it can also create problems. For example, sometimes one project needs the latest version of Django, while another project needs an older Django version to retain compatibility with a critical third-party extension. This is one of many use cases that virtualenv was designed to solve. On my systems, only a handful of general-purpose Python packages (such as Mercurial and virtualenv) are globally available — every other package is confined to virtual environments.

With that explanation behind us, let’s install virtualenv:

pip install virtualenv

Create some directories to store our projects and virtual environments, respectively:

mkdir -p ~/Projects ~/Virtualenvs

We’ll then open the ~/.bashrc file (which may be created if it doesn’t exist yet)…

vim ~/.bashrc

… and add some lines to it:

# pip should only run if there is a virtualenv currently activated
export PIP_REQUIRE_VIRTUALENV=true
# cache pip-installed packages to avoid re-downloading
export PIP_DOWNLOAD_CACHE=$HOME/.pip/cache

Let’s re-load our bash environment:

source ~/.bash_profile

Now we have virtualenv installed and ready to create new virtual environments, which we will store in ~/Virtualenvs. New virtual environments can be created via:

cd ~/Virtualenvs
virtualenv foobar

If you have both Python 2.x and 3.x and want to create a Python 3.x virtualenv:

virtualenv -p python3 foobar-py3

… which makes it easier to switch between Python 2.x and 3.x foobar environments.

Restricting pip to virtual environments

What happens if we think we are working in an active virtual environment, but there actually is no virtual environment active, and we install something via pip install foobar? Well, in that case the foobar package gets installed into our global site-packages, defeating the purpose of our virtual environment isolation.

In an effort to avoid mistakenly pip-installing a project-specific package into my global site-packages, I previously used easy_install for global packages and the virtualenv-bundled pip for installing packages into virtual environments. That accomplished the isolation objective, since pip was only available from within virtual environments, making it impossible for me to pip install foobar into my global site-packages by mistake. But easy_install has some deficiencies, such as the inability to uninstall a package, and I found myself wanting to use pip for both global and virtualenv packages.

Thankfully, pip has a sparsely-documented setting that tells it to bail if there is no active virtual environment, which is exactly what I want. In fact, we’ve already set that above, via the export PIP_REQUIRE_VIRTUALENV=true directive. For example, let’s see what happens when we try to install a package in the absence of an activated virtual environment:

$ pip install markdown
Could not find an activated virtualenv (required).

Perfect! But once that option is set, how do we install or upgrade a global package? We can temporarily turn off this restriction by adding the following to your ~/.bashrc:

gpip(){
   PIP_REQUIRE_VIRTUALENV="" pip "$@"
}

(As usual, after adding the above you must run source ~/.bash_profile for the change to take effect.)

If in the future we want to upgrade our global packages, the above function enables us to do so via:

gpip install --upgrade --no-use-wheel pip setuptools virtualenv

You could, of course, do the same via PIP_REQUIRE_VIRTUALENV="" pip install --upgrade foobar, but that’s much more cumbersome to type.

Creating virtual environments

Let’s create a virtual environment for Pelican, a Python-based static site generator:

cd ~/Virtualenvs
virtualenv pelican

Change to the new environment and activate it via:

cd pelican
. bin/activate

To install Pelican into the virtual environment, we’ll use pip:

pip install pelican markdown

For more information about virtual environments, read the virtualenv docs.

Dotfiles

These are obviously just the basic steps to getting a Python development environment configured. If you found this useful, you might to also check out my dotfiles (GitHub mirror), which I recently started rebuilding from scratch. I’m still early in the process of selectively adding one building block at a time, with more to be added soon.

Thoughts?

Have a question or suggestion? Leave a comment below!

Follow me on Twitter to be notified when new articles are posted. You can also follow me on GitHub.

Comments