I downloaded Python 3.5 and tried opening the IDLE shell. 

Each time I do it, it says "The application "IDLE" can't be opened."

Any help? My Mac OS X is 10.14.6 Mojave

Python 3.5 is very old; the last macOS installers from python.org for it were released over 3 years ago and the 3.5 release series has reached end-of-life.  Is there a reason you can't use a current version of Python?  3.8.6 is now current. Since Python 3.5.4, the last 3.5 installers, there have been a number of important security fixes and support for current versions of macOS.

Hey Ned, I'm using 3.5 in accordance with what "Introduction to Computation and Programming Using Python|With Application to Understanding Data" 

Should I just uninstall 3.5 and opt to use 3.8.6?

Ah! I've never looked at the book or the MIT course materials.  I see that an update to the book is planned for next year and that 3.5 would have been current when the current edition of the book was published. Generally, changes between recent Python feature releases are upward compatible. Off the top of my head, I can't think of any changes at the Python source level that would cause examples written for 3.5.x to fail on 3.8.x. Perhaps you could ask around online but, rather than trying to debug what's going wrong with your 3.5.x installation, I think you would be better off with 3.8.6. You can just install it on top of 3.5.x. Good luck!

Thank you very much! I will uninstall soon and download 3.8.6 -- if anything comes up then I'll let you know fosho

To try to run IDLE with 8.5, one should install tcl/tk 8.5.18 as described at https://www.python.org/download/mac/tcltk/.  This requires making a free Active State account and is not at all guaranteed to be sufficient.

Today I installed 3.8.6 and 3.9.0rc2 (3.9.0 is due in 2 weeks) on an Airbook with up to date Mohave and ran IDLE. In both cases, there are a few Mac-specific bugs, but IDLE mostly works.

Thank you both. Decided to just download 3.8.6 instead of dealing with 3.5

IDLE shell works. Appreciate the help.