In msg225377 of #17535, Ned Daily says "(Noted in passing: while the help/doc suggests: "See the beginning of config-extensions.def in the idlelib directory for further information.", even in the unlikely event that a user knew in what directory to look for it, it's not possible to open that file in an IDLE editor window with the current default Cocoa Tk's since Cocoa Tk does not provide a filter option on the open window; only .py* and .txt files can be opened.)"

Users should not normally poke around idlelib and should not need to or be asked to.  The extension information should either be added to the idle doc or make accessible on the help menu with an new 'Extensions' entry. If the docstring were moved to, say, configHandler.py, it would be trivial to access: 'from configHandler import __doc__ and help_extension".  Since configHandler already knows how to find the file, we can just read it up to the second """ line.

Note: regardless of the resolution of this issue, show_text() should be altered to start wide enough to show 80 char lines.

Open on macOS Mohave now has an All files * filter option, and opening *.def is easy.  If not true on every Mac still getting new versions of Python, too bad.