traceback.print_last behaving differently across python versions

[mironleon]

Bug report

Bug description:

traceback.print_last() correctly prints the latest traceback for python 3.10.16 and 3.11.9, but it prints NoneType: None for 3.12.9 and 3.13.2

See also ipython/ipython#14744, I originally thought it was an ipython issue

Python 3.10.16 (main, Feb 13 2025, 14:32:36) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> import traceback
>>> traceback.print_last()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

Python 3.11.9 (main, Feb 13 2025, 14:34:41) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> import traceback
>>> traceback.print_last()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

Python 3.12.9 (main, Feb 13 2025, 13:23:03) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> import traceback
>>> traceback.print_last()
NoneType: None

Python 3.13.2 (main, Feb 13 2025, 11:10:27) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 1/0
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    1/0
    ~^~
ZeroDivisionError: division by zero
>>> import traceback
>>> traceback.print_last()
NoneType: None

CPython versions tested on:

3.10, 3.11, 3.12, 3.13

Operating systems tested on:

Linux

Linked PRs

• gh-130250: fix regression in traceback.print_last #130318
• [3.13] gh-130250: fix regression in traceback.print_last (GH-130318) #130325
• [3.12] gh-130250: fix regression in traceback.print_last (GH-130318) #130326
• gh-130250: fix cleanup in test (impacts refleaks runs) #130329
• [3.13] gh-130250: fix cleanup in test (impacts refleaks runs) (GH-130329) #130330
• [3.12] gh-130250: fix cleanup in test (impacts refleaks runs) (GH-130329) #130331
• gh-130250: use support.swap_attr() in test and don't assume we're beginning with a clean system state #130342
• [3.13] gh-130250: use support.swap_attr() in test and don't assume we're beginning with a clean system state (GH-130342) #130344
• [3.12] gh-130250: use support.swap_attr() in test and don't assume we're beginning with a clean system state (GH-130342) #130345

[serhiy-storchaka]

If you remove sys.last_exc, you will get the old behavior.

>>> import sys
>>> sys.last_exc
ZeroDivisionError('division by zero')
>>> del sys.last_exc
>>> traceback.print_last()
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    1/0
    ~^~
ZeroDivisionError: division by zero

The regression was most likely introduced in #102778. cc @iritkatriel