Error on Python 3.13 and newer.

Reproducer

• Run IDLE
• Set a custom sys.excepthook:

import sys
def hook(*args): return sys.__excepthook__(*args)
sys.excepthook=hook

• Trigger an error which display a traceback, such as accessing a variable which doesn't exist.

Output

>>> os
Traceback (most recent call last):
  File �[35m"/home/vstinner/python/3.13/Lib/idlelib/run.py"�[0m, line �[35m590�[0m, in �[35mruncode�[0m
    �[31mexec�[0m�[1;31m(code, self.locals)�[0m
    �[31m~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^�[0m
  File �[35m"<pyshell#12>"�[0m, line �[35m1�[0m, in �[35m<module>�[0m
�[1;35mNameError�[0m: �[35mname 'os' is not defined. Did you forget to import 'os'?�[0m

ANSI sequences to colorize the output are unexpected and makes the output hard to read.

Expected output

>>> os
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    os
NameError: name 'os' is not defined. Did you forget to import 'os'?

Explanation

In the normal case:

• IDLE replaces sys.stderr with io.StringIO() to call the "excepthook".
• _colorize.can_colorize() returns False.
• It works as expected.

If sys.excepthook is overridden:

• IDLE replaces sys.stderr with a idlelib.run.StdOutputFile object at startup.
• _colorize.can_colorize() returns True because StdOutputFile.isatty() always return True.

Linked PRs

• gh-127060: IDLE StdOutputFile is no longer a TTY #127061
• gh-127060: Disable traceback colors in IDLE #128028
• [3.13] gh-127060: Disable traceback colors in IDLE (GH-128028) #128052