I have Python 2.x and 3.x on my machine (Mac OS X 10.6). For some things I want to use ver 2, but for others I want ver 3. I like the IDLE software for editing/running, but
Usually each Python version installs its own version of IDLE. I don't know how this works on Mac, but for Windows it works like this:
python2.7 C:\Program Files\Python27\Lib\idlelib\idle.pyw
starts the Python 2 IDLE and
python3.2 C:\Program Files\Python32\Lib\idlelib\idle.pyw
starts the Python 3 IDLE.