Multiple Homebrew Pythons

The Homebrew project brings a really nice package installer to OS X, allowing you to install command line programs in a really simple way. For instance, to install the latest version of Python 3, you would do:

$ brew install python3

Because not all projects are as aware of old versions as python, when brew upgrades a package, it removes the old versions linked binaries and support files. This is actually not a good thing for python: it means you can no longer access the older interpreter.

Python keeps version-named interpreters, and then just symlinks the most recently installed to the python executable. Thus, it’s not uncommon to see, for python2:

$ ls -1 /usr/bin/python*

This means, if you want to run an older version (for instance, say you use tox and want to do some testing against a range of versions), you can just use:

$ python2.5
Python 2.5.6 (r256:Unversioned directory, Mar  9 2014, 22:15:03) 
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

But Homebrew breaks this.

However, if you have the old versions already installed, you can easily recreate the symlinks. Indeed, here is a script that will visit all homebrew installed python3 versions, creating

cd /usr/local/Cellar/python3/

for VERSION in `ls`
  find bin -name \*3\* -exec ln -s -f `pwd`/{} /usr/local/{} \;
  find lib -name \*python\* -maxdepth 1 -exec ln -s -F `pwd`/{} /usr/local/{} \;
  find share -name python\* -exec ln -s -f `pwd`/{} /usr/local/{} \;
  find Frameworks -name 3.\* -exec ln -s -f `pwd`/{} /usr/local/{} \;
  cd ..

It worked for me for python3 with the following versions installed:

  • 3.2.3
  • 3.3.3
  • 3.4.1

Now I just need to figure out how to get Homebrew to download and build specific versions of packages.