Per Martin's request, this issue has been separated from Issue14200. 

The IDLE shell presently has an effective locale of "BMP UTF8" due to a limitation in Tkinter, described in Issue12342. 

IDLE should support different output codecs, like "ascii" and "utf8". In order to work around Tkinter's limitations, unsupported characters would be replaced (this is related to #14304).

This amounts to adding some extra code to OutputWindow's write() to raise encoding errors if the string contains unsupported characters.