The main use for the iomenu settings is for the socket-transport file classes, in run.py.  The default encoding='utf-8' and errors='strict' are not used but are overriden with the iomenu values, or for stderr, 'backslashreplace'.

Since user code can print any unicode, I think the defaults should used as is to transparently pass on and possibly display anything the user sends.  Such a change should have no back-compatibility issues.

Thinking more about errors.  With utf-8 encoding of proper strings, there should never be any, but Python does allow construction of 'improper' strings with, say, single surrogates.  The transport mechanism should never raise, so maybe surrogateescape or backslashreplace should always be used. 

What do you two think?

Another use is for writing bytes to an OutputWindow, as with find-in-files.  But I can think of no case where IDLE sends bytes to an OutputWindow.  User files are all opened in an editor.

I believe these are all the uses of 'iomenu.encoding' outside of iomenu.  'from iomenu ...' is never used.

Within iomenu, the only use is part of reading an encoding cookie.
    # The only use of 'encoding' below is in _decode as initial value
    # of deprecated block asking user for encoding.
I am not sure if this use can be reached now.  Even if so, I believe this code duplicates code elsewhere in the stdlib that might be used.

So maybe the encoding calculation is not really needed.