Mountain Lion deleted all my Python packages, just like Lion did last year. I’m determined not to be fooled a third time, so this time I installed my packages to a custom directory using virtualenv.

Virtualenv is advertised as a way to create multiple isolated Python environments on the same computer, switch between them easily, etc. I don’t need all of that. I just want to control where Python packages are installed with no fuss. Virtualenv does that also.

Set up a virtual environment

First, install virtualenv. I recommend installing pip and running pip install virtualenv. If you think it is already installed but it isn’t working, this is probably because Mountain Lion deleted it with the rest of your packages. Reinstall virtualenv.

Second, run virtualenv /usr/local/python. This creates a new Python environment based at /usr/local/python. Of course, you can make this any path you want.

You now have two new directories (among others):

/usr/local/python/bin
/usr/local/python/lib/python2.7/site-packages

The site-packages directory is where your packages will be installed. The bin directory contains a special pip executable, which automatically installs new packages to your custom directory. It also contains a python executable, which will make these packages available to a Python session. Also, if you install any package that has a command line interface, the executable file will go in this bin directory. I added /usr/local/python/bin to (the front of) my PATH to make these easily accessible.

Make these packages available to the system’s Python

Third, create a file local.pth (name doesn’t matter, but extension does) inside the /Library/Python/2.7/site-packages folder with a single line:

/usr/local/python/lib/python2.7/site-packages

This tells the regular system Python to also load packages from my custom location. So even if I run /usr/bin/python, I will be able to import my packages.

As long as I always use the pip command that virtualenv created to install new packages, this third step is the only thing I will have to repeat next year, when OS X 10.9 “Volcano Lion” clears out the system site-packages folder again.