Python IDLE: Change Python Version

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.

If there are 2 or more Python versions installed on you computer but you can only open one Python IDLE (suppose IDLE using Python 2.7) and if you dont have IDLE for Python 3.5 installed on your computer (check the folder /usr/bin for idle or idle-python2.7 And if idle-python3.5 is not there then its not currently installed). Then you just have to install idle-python3.5 by executing following command through CLI.

sudo apt install idle-python3.5

Or you can just install it by Software Manager in Linux. Search for IDLE and select your preferred IDLE to install.

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

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.