#14576 is more or less about making better use of USERPROFILE when the value returned by expanduser to config.IdleConf.GetUserCfgDir fails.  Now that I know that tcl always sets HOME (or pieces that are joined back together to make HOME), so that expanduser never fails (?), this might make some sense - but only if they are not the same.

Zach, I was thinking that _tkinter read TCL_LIBRARY to start tcl, but your answer suggests that it is instead read by tcl.

The problem with bad TCL_LIBRARY is that if IDLE is started normally, there is no error message.  Even if the user tries starting in the console, the message is almost useless.  I "set TCL_LIBRARY=NONE" and got
   ...
  File "C:\Programs\Python35\lib\tkinter\__init__.py", line 1867, in __init__
    self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
_tkinter.TclError: Can't find a usable init.tcl in the following directories:
    NONE C:/Programs/Python35/lib/tcl8.6 C:/Programs/lib/tcl8.6 C:/lib/tcl8.6 C:/Programs/library C:/library C:/tcl8.6.4/library C:/tcl8.6.4/library

Everything after NONE is nonsensical.  The IDLE doc needs a new sections "If IDLE does not start:.  Now I could write a troubleshooting note explaining for this note to ignore everything after the first item, and that the first item is an unusable setting of TCL_LIBRARY set by something or someone other than Python/Tkinter/IDLE.

I tried the same experiment with TK_LIBRARY and could see no effect.