Broken "Exception ignored in:" message on exceptions in repr #67025

The-Compiler · 2014-11-10T10:22:49Z

BPO	22836
Nosy	@salty-horse, @vstinner, @bitdancer, @vadmium, @serhiy-storchaka, @The-Compiler
Files	repr_exception.py: Script demonstrating the issue unraisable-continue.patch unraisable-continue.v2.patch unraisable-continue.v3.patch unraisable-continue.v4.patch unraisable-continue.v5.patch

^{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 = 'https://github.com/vadmium'
closed_at = <Date 2016-02-28.04:17:57.421>
created_at = <Date 2014-11-10.10:22:49.203>
labels = ['interpreter-core', 'type-bug']
title = 'Broken "Exception ignored in:" message on exceptions in __repr__'
updated_at = <Date 2019-01-15.10:23:48.960>
user = 'https://github.com/The-Compiler'

bugs.python.org fields:

activity = <Date 2019-01-15.10:23:48.960>
actor = 'salty-horse'
assignee = 'martin.panter'
closed = True
closed_date = <Date 2016-02-28.04:17:57.421>
closer = 'martin.panter'
components = ['Interpreter Core']
creation = <Date 2014-11-10.10:22:49.203>
creator = 'The Compiler'
dependencies = []
files = ['37166', '37511', '37524', '39670', '41948', '42029']
hgrepos = []
issue_num = 22836
keywords = ['patch']
message_count = 18.0
messages = ['230950', '230955', '232961', '233001', '233005', '245104', '260379', '260385', '260395', '260434', '260765', '260778', '260779', '260855', '260953', '260963', '260964', '333663']
nosy_count = 8.0
nosy_names = ['salty-horse', 'vstinner', 'Arfrever', 'r.david.murray', 'python-dev', 'martin.panter', 'serhiy.storchaka', 'The Compiler']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22836'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

The-Compiler · 2014-11-10T10:22:49Z

When there's an unraisable exception (e.g. in __del__), and there's an exception in __repr__ as well, PyErr_WriteUnraisable returns after writing "Exception ignored in:" immediately.

I'd expect it to fall back to the default __repr__ instead.

See the attached example script.

Output with 3.4:

=== Obj ===

Exception ignored in: <bound method Obj.__del__ of <__main__.Obj object at 0x7fd842deb4a8>>
Traceback (most recent call last):
  File "test.py", line 4, in __del__
    raise Exception('in del')
Exception: in del
=== BrokenObj

===
Exception ignored in: (no newline)

Output with 2.7:

=== Obj ===
Exception Exception: Exception('in del',) in <bound method Obj.__del__ of <main.Obj object at 0x7fa824dbfa50>> ignored
=== BrokenObj ===
Exception Exception: Exception('in del',) in ignored

The output with 2.7 is a bit more useful, but still confusing.

vadmium · 2014-11-10T11:17:40Z

This is one that has often bugged me. When your repr() implementation is broken, it is quite confusing figuring out what is going wrong. Falling back to object.__repr__() is one option, however I would probably be happy with a simple “exception in repr()” message, and a proper newline.

Another way that I have come across this is:

$ python -c 'import sys; sys.stdout.detach()'
Exception ignored in: [no newline]

The workaround there is to set sys.stdout = None. In that case I think repr(sys.stdout) is trying to say “ValueError: underlying buffer has been detached”.

vadmium · 2014-12-20T04:18:36Z

Here is a patch that substitutes an explanation if the repr() fails. Output now looks like this, terminated with a newline:

=== BrokenObj ===

Exception ignored in: <repr() failed>
Traceback (most recent call last):
  File "<stdin>", line 3, in __del__
Exception: in del
$ ./python -c 'import sys; sys.stdout.detach()'
Exception ignored in: <repr() failed>
ValueError: underlying buffer has been detached

I also made it work sensibly if printing the exception message fails:

>>> class Exception(Exception):
...     def __str__(self): raise Exception("Exception is broken")
... 
>>> f = BrokenObj(); del f
Exception ignored in: <repr() failed>
Traceback (most recent call last):
  File "<stdin>", line 3, in __del__
__main__.Exception: <str() failed>
>>> raise Exception()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.Exception: <str() failed>
>>>

bitdancer · 2014-12-21T18:24:26Z

See also bpo-6294 for a related problem.

vadmium · 2014-12-21T23:34:16Z

Patch v2 revises the unit tests so they are cleaner. Also now tests that the <repr() failed> placeholders are in the exception reports.

vadmium · 2015-06-10T02:02:04Z

Updated the patch to address an oversight in the new test cases.

I think bpo-6294 is probably the same underlying issue. The patch there could be used as the basis for a Python 2 patch. My patches here are for Python 3, and I suspect the code is significantly different in each version, because the error messages are different.

Comparison (for bikeshedding etc) with the message produced by the “traceback” module when str() fails:

>>> try:
...     raise BrokenStrException("message")
... except BrokenStrException:
...     print_exc()
...     raise
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
BrokenStrException: <unprintable BrokenStrException object>
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
__main__.BrokenStrException: <str() failed>

The-Compiler · 2016-02-17T08:14:14Z

I just got bitten by this again - is anything holding this back from being merged (other than lack of review-manpower)?

vadmium · 2016-02-17T10:11:35Z

Yes a review would help. Also, I suspect this will require a separate patch for Python 2. When I get a chance I will have another look at this and see if I am comfortable committing it.

serhiy-storchaka · 2016-02-17T11:47:07Z

This may be a part of more general issue. Should repr() fail at all? Wouldn't be better to fall back to the default __repr__ instead? repr() is typically used for debugging. Failing repr() can be a part of larger __repr__, thus raising an exception in subobject's repr() leads to the loss of information about full object.

vadmium · 2016-02-18T06:55:27Z

Thanks for the review; here is an updated patch.

If we just fall back the the default, it hides the fact that that there is a problem in the custom __repr__() method. Another option may be to both indicate there was an error, _and_ a fall back.

serhiy-storchaka · 2016-02-24T08:16:53Z

Surely changing the behavior of repr() can be made only in the default branch. this issue needs the patch for all versions.

I don't know whether this is the best solution, but the patch looks good enough to me at this moment. If you have no doubts feel free to commit it Martin.

vstinner · 2016-02-24T09:54:29Z

I reviewed unraisable-continue.v4.patch, comments on Rietveld.

Would it be possible to include at least the name of the exception type in the error message? It's less likely than writing Py_TYPE(obj)->tp_name failed, than error in exc.__str() no?

vstinner · 2016-02-24T09:57:15Z

I like the idea of enhance code writing exception reports, I'm strongly opposed to modify repr() to ignore exceptions. I would to be noticed when repr() fails badly.

I like how the logging module catchs any formating error and logs an error when the formatting fails.

vadmium · 2016-02-25T13:04:03Z

Here is a new patch with Victor’s suggestions. The reports now look like

>>> raise BrokenStrException()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.BrokenStrException: <exception str() failed>
>>> o = VeryBroken()
>>> del o
Exception ignored in: <object repr() failed>
Traceback (most recent call last):
  File "<stdin>", line 9, in __del__
__main__.BrokenStrException: <exception str() failed>

python-dev · 2016-02-28T01:19:42Z

New changeset fca9f02e10e5 by Martin Panter in branch '2.7':
Issue bpo-22836: Keep exception reports sensible despite errors
https://hg.python.org/cpython/rev/fca9f02e10e5

python-dev · 2016-02-28T03:45:04Z

New changeset cf70b1204e44 by Martin Panter in branch '3.5':
Issue bpo-22836: Keep exception reports sensible despite errors
https://hg.python.org/cpython/rev/cf70b1204e44

New changeset 2b597e03f7f4 by Martin Panter in branch 'default':
Issue bpo-22836: Merge exception reporting from 3.5
https://hg.python.org/cpython/rev/2b597e03f7f4

vadmium · 2016-02-28T04:17:57Z

FTR Python 2’s exception report in __del__() is a bit different, here is what it now looks like:

>>> o = VeryBroken()
>>> del o
Exception __main__.BrokenStrException: <exception repr() failed> in <object repr() failed> ignored

salty-horse · 2019-01-15T10:23:49Z

I encountered a similar bug and reported it as bpo-35743.

The-Compiler mannequin added interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error labels Nov 10, 2014

serhiy-storchaka assigned vadmium Feb 24, 2016

vadmium closed this as completed Feb 28, 2016

ezio-melotti transferred this issue from another repository Apr 10, 2022

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Broken "Exception ignored in:" message on exceptions in repr #67025

Broken "Exception ignored in:" message on exceptions in repr #67025

The-Compiler mannequin commented Nov 10, 2014

The-Compiler mannequin commented Nov 10, 2014

vadmium commented Nov 10, 2014

vadmium commented Dec 20, 2014

bitdancer commented Dec 21, 2014

vadmium commented Dec 21, 2014

vadmium commented Jun 10, 2015

The-Compiler mannequin commented Feb 17, 2016

vadmium commented Feb 17, 2016

serhiy-storchaka commented Feb 17, 2016

vadmium commented Feb 18, 2016

serhiy-storchaka commented Feb 24, 2016

vstinner commented Feb 24, 2016

vstinner commented Feb 24, 2016

vadmium commented Feb 25, 2016

python-dev mannequin commented Feb 28, 2016

python-dev mannequin commented Feb 28, 2016

vadmium commented Feb 28, 2016

salty-horse mannequin commented Jan 15, 2019

Broken "Exception ignored in:" message on exceptions in __repr__ #67025

Broken "Exception ignored in:" message on exceptions in __repr__ #67025

Comments

The-Compiler mannequin commented Nov 10, 2014

The-Compiler mannequin commented Nov 10, 2014

vadmium commented Nov 10, 2014

vadmium commented Dec 20, 2014

bitdancer commented Dec 21, 2014

vadmium commented Dec 21, 2014

vadmium commented Jun 10, 2015

The-Compiler mannequin commented Feb 17, 2016

vadmium commented Feb 17, 2016

serhiy-storchaka commented Feb 17, 2016

vadmium commented Feb 18, 2016

serhiy-storchaka commented Feb 24, 2016

vstinner commented Feb 24, 2016

vstinner commented Feb 24, 2016

vadmium commented Feb 25, 2016

python-dev mannequin commented Feb 28, 2016

python-dev mannequin commented Feb 28, 2016

vadmium commented Feb 28, 2016

salty-horse mannequin commented Jan 15, 2019

Broken "Exception ignored in:" message on exceptions in repr #67025

Broken "Exception ignored in:" message on exceptions in repr #67025