Python IDLE: Change Python Version
There are different versions of IDLE installed for each Python version. Depending on how you installed Python on Mac OS X, you may find different folders in /Applications. Look for a Python 3.n (n = 1 or 2) folder with an IDLE in it. Or, from a terminal command line, you may find an idle2.6 and an idle3 or idle3.1 or idle3.2.
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.pywstarts the Python 2 IDLE and
python3.2 C:\Program Files\Python32\Lib\idlelib\idle.pywstarts the Python 3 IDLE.
In the Windows environment, if you want to use particular install of IDLE, I find the easiest way is to right click on the .py file and choose "open with". Then navigate to the IDLE.bat file in the Python version install location you want to use. I find picking just the ilde.py doesn't work but the .bat file does.
E. g.
D:\Python27\ArcGISx6410.4\Lib\idlelib\idle.bat
or
D:\Python27Desktop\ArcGIS10.4\Lib\idlelib\idle.bat