There is a need for IDLE to provide different default settings depending on which platform it is running, e.g. Windows, OS X, or other UNIX.  That need need has been most acute for OS X where, for various reasons, tailoring of default key bindings for IDLE commands and extensions is either necessary or desirable to conform to platform user interface guidlines or, more importantly, platform-specific Tk behavior.  For OS X, that customization is provided in part by brute force editing of the config-main.def and config-extensions.def in Mac/Makefile target install_IDLE which is run during installation of framework builds.  While that works for that case, it is brittle and opaque and does not help for non-framework OS X installs or when running from a build directory.  And it doesn't help for other platforms.  Other OS X customization is done at run time to convert any <Alt-Key-> events to <Option-Key-> events, since none of the OS X Tk implementations support <Alt-Key-> events (see Issue20579).  It would be better to provide some way to specify these various conditional settings directly in config-main.def and config-extensions.def themselves, perhaps with platform-specific sections in the files that would be merged with and override the current general sections.  For example, in config-main.def, a possible implementation might be:

[Keys]
default= 1
name= IDLE Classic Windows

[Keys-OSX]
name= IDLE Classic OSX

[Keys-Unix]
name= IDLE Classic Unix

This might also be used to support different options for different versions of IDLE since today all versions by default share the same user configuration files.

[Keys]
...

[Keys-2.7]
...

[Keys-3]
...