Which Version of Python Do You Already Have Installed on Mac OS X?

Open the Terminal.app and type which python,  and on a fresh install of Snow Leopard you should see:

/usr/bin/python

This is a symlink that points to your actual Python installation.

Next, type cd /usr/bin; ls -l python2.6, and the output should be something like:

lrwx-xr-x 1 root wheel 75 Oct 7 23:24 python2.6 ->
../../System/Library/Frameworks/Python.framework/Versions/2.6/bin/Python2.6

Now open an interactive interpreter session:

Python 2.61 (r261:67515, Feb 11 2010, 00:51:29)
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits", or "license" for more information.
>>>