OS X Leopard (1.5.8) came with pre-installed Python 2.5, so, when you trying to add Python interpreters on Aptana Studio 3 by clicking the “Auto Config” button, you will get Python 2.5… of course :-p

So, download the lates Python for Mac OS X from here: www.python.org/download/ , and then install it.

Add the latest Python interpretes on Aptana Studio 3 by going to Preferences (Cmd+,) -> PyDev -> Interpreter – Pyhton .
Click “New” button, then click the “Browse…” button, and locate the Interpreter Executable to: /Library/Frameworks/Python.framework/Versions/X.Y/bin/pythonX.Y (X.Y is the version of the Python), hit the “OK” button… and you are done.