You meant it the other way around 'replace sys.stderr.write() by print(file=sys.stderr)'.  In the two cases where Idle exits anyway, this will not make any difference (except possible in the return code).  In the other cases where Idle tries to continue, this will.  The real fix needed is to replace stderr, at least when None, so that the messages are displayed in a gui widget.  However, if I cannot do that this week, before 3.4.2, I will apply this. Thanks for the patch.