3.10a5 regression: AttributeError: 'NoneType' object has no attribute '__suppress_context__' in traceback.py

[The-Compiler]

BPO	43146
Nosy	@gvanrossum, @terryjreedy, @vsajip, @The-Compiler, @hroncok, @ZackerySpytz, @pablogsal, @iritkatriel
PRs	bpo-43146: fix a recent (3.10) regression in handling None inputs to `tra… #24463 bpo-43146: fix None-handling in single-arg version of traceback.print… #24629

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-02-23.19:39:25.530>
created_at = <Date 2021-02-06.15:54:16.813>
labels = ['type-bug', 'library', '3.10', 'release-blocker']
title = "3.10a5 regression: AttributeError: 'NoneType' object has no attribute '__suppress_context__' in traceback.py"
updated_at = <Date 2021-02-23.19:39:25.530>
user = 'https://github.com/The-Compiler'

bugs.python.org fields:

activity = <Date 2021-02-23.19:39:25.530>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-23.19:39:25.530>
closer = 'iritkatriel'
components = ['Library (Lib)']
creation = <Date 2021-02-06.15:54:16.813>
creator = 'The Compiler'
dependencies = []
files = []
hgrepos = []
issue_num = 43146
keywords = ['patch', '3.10regression']
message_count = 18.0
messages = ['386562', '386563', '386568', '386796', '386885', '386886', '386917', '386928', '387159', '387573', '387574', '387575', '387579', '387580', '387581', '387582', '387586', '387587']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'terry.reedy', 'vinay.sajip', 'The Compiler', 'hroncok', 'ZackerySpytz', 'pablogsal', 'iritkatriel']
pr_nums = ['24463', '24629']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43146'
versions = ['Python 3.10']

[The-Compiler]

After upgrading to 3.10a5, calling logging.exception("test") results in:

--- Logging error \---

    Traceback (most recent call last):
      File "/usr/lib/python3.10/logging/__init__.py", line 1094, in emit
        msg = self.format(record)
      File "/usr/lib/python3.10/logging/__init__.py", line 938, in format
        return fmt.format(record)
      File "/usr/lib/python3.10/logging/__init__.py", line 682, in format
        record.exc_text = self.formatException(record.exc_info)
      File "/usr/lib/python3.10/logging/__init__.py", line 632, in formatException
        traceback.print_exception(ei[0], ei[1], tb, None, sio)
      File "/usr/lib/python3.10/traceback.py", line 113, in print_exception
        te = TracebackException(type(value), value, tb, limit=limit, compact=True)
      File "/usr/lib/python3.10/traceback.py", line 531, in __init__
        need_context = cause is None and not e.__suppress_context__
    AttributeError: 'NoneType' object has no attribute '__suppress_context__'

That method is documented as "This function should only be called from an exception handler.", but the same also happens when e.g. calling traceback.print_exc() (without a current exception) or traceback.print_exception(None, None, None). Both worked fine before 3.10 Alpha 5, the code seems to do None-checks in other places, and typeshed also marks those arguments as Optional.

This seems to have been introduced in #24179 for bpo-42877.

[iritkatriel]

Thanks, I’ll add the None check.

[iritkatriel]

There's another such issue due to PR 22610 // bpo-26389:

>>> traceback.format_exception(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython-dev\lib\traceback.py", line 128, in format_exception
    value, tb = _parse_value_tb(exc, value, tb)
  File "C:\Users\User\src\cpython-dev\lib\traceback.py", line 94, in _parse_value_tb
    return exc, exc.__traceback__
AttributeError: 'NoneType' object has no attribute '__traceback__'

[iritkatriel]

Marking as blocker because these are 3.10 regressions.

[terryjreedy]

It seems to me that print_exception(None), etc, *should* raise something. Printing "NoneType: None\n" makes no sense to me since NoneType is not an exception. In 3.9, it raised TypeError for # of arguments.

I do note that in 3.9
>>> traceback.print_exception(None, None, None)
NoneType: None

I wonder what the rationale was. It isn't because these functions never raise.

[iritkatriel]

In 3.9 you had to give exc,val,tb. In 3.10 the shortcut was added. bpo-26389.

[hroncok]

JFYI, there are 2 affected Fedora packages (that we know of):

visidata fails to build with Python 3.10: AttributeError: 'NoneType' object has no attribute '__suppress_context__'
https://bugzilla.redhat.com/show_bug.cgi?id=1928145

python-utils fails to build with Python 3.10: AttributeError: 'NoneType' object has no attribute '__suppress_context__'
https://bugzilla.redhat.com/show_bug.cgi?id=1928081

[iritkatriel]

I wonder what the rationale was. It isn't because these functions never raise.

I think it's because sys.exc_info() can return None, None, None when there is no exceptions.

[iritkatriel]

Let's split the print_exception(None) case (new issue) from the regression. I've updated the PR to fix only the latter, which is not controversial.

I don't know what print_exception(None) should do.

[gvanrossum]

New changeset 26f18b8 by Irit Katriel in branch 'master':
bpo-43146: fix regression in traceback.print_exception(None) (GH-24463)
26f18b8

[gvanrossum]

Are you going to create a separate PR or a new issue for the other thing? (And what exactly *is* the other thing? Please summarize.)

[iritkatriel]

The other thing is not related to the one reported here, but I found it while writing the test for this.

We have a new API in 3.10, where you can do traceback.print_exception(ex) (so you don't need to give the whole type,val,tb triplet).

But:

>>> traceback.print_exception(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\User\src\cpython-dev\lib\traceback.py", line 110, in print_exception
    value, tb = _parse_value_tb(exc, value, tb)
  File "C:\Users\User\src\cpython-dev\lib\traceback.py", line 94, in _parse_value_tb
    return exc, exc.__traceback__
AttributeError: 'NoneType' object has no attribute '__traceback__'

[gvanrossum]

So the PR just merged fixes that, right? Color me confused.

[iritkatriel]

The PR just merged fixes the issue that was reported by Florian (what I broke). That impacted pre-3.10 APIs so was an actual regression.

The issue with traceback.print_exception(None) is new to 3.10, and was introduced in bpo-26389.

[gvanrossum]

So the remaining question is what to do for print_exception(None)? Honestly
the current (new) behavior of printing NoneType: None seems fine.

[iritkatriel]

So the remaining question is what to do for print_exception(None)?

Yes.

Honestly the current (new) behavior of printing NoneType: None seems fine.

That's not the current behaviour - I removed that fix from the PR till we agree what to do. Currently we get the AttributeError.

[iritkatriel]

The new PR makes it print NoneType: None, which is at least consistent with what the same functions do without the bpo-26389 shortcut.

[gvanrossum]

New changeset b798ab0 by Irit Katriel in branch 'master':
bpo-43146: fix None-handling in single-arg traceback.print_exception(None) (GH-24629)
b798ab0