pickle does not serialize Exception __cause__ field

@brettcannon

BPO	29466
Nosy	@brettcannon, @ncoghlan, @diekhans, @avassalotti, @ericsnowcurrently, @serhiy-storchaka, @tjb900, @kakshay21, @iritkatriel
Files	cause_pickle.py: bug demog program

^{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 = None
created_at = <Date 2017-02-06.18:36:36.651>
labels = ['type-bug', 'library', '3.11']
title = 'pickle does not serialize Exception __cause__ field'
updated_at = <Date 2022-01-06.16:21:58.240>
user = 'https://github.com/diekhans'

bugs.python.org fields:

activity = <Date 2022-01-06.16:21:58.240>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-02-06.18:36:36.651>
creator = 'diekhans'
dependencies = []
files = ['46550']
hgrepos = []
issue_num = 29466
keywords = []
message_count = 6.0
messages = ['287163', '287304', '335686', '396603', '396604', '409871']
nosy_count = 9.0
nosy_names = ['brett.cannon', 'ncoghlan', 'diekhans', 'alexandre.vassalotti', 'eric.snow', 'serhiy.storchaka', 'tjb900', 'kakshay', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29466'
versions = ['Python 3.11']

python3 pickle does not serialize the __cause__ field, as shown by the attached demo program.

True. Attributes __context__, __cause__ and __traceback__ are not pickled. The traceback objects are even not pickleable.

What is worse, some other non-special attributes are lost during pickling. For example name and path attributes of ImportError.

>>> try: import foo
... except Exception as ex: exc = ex
... 
>>> exc.name
'foo'
>>> exc.__reduce__()
(<class 'ModuleNotFoundError'>, ("No module named 'foo'",), {})

Or the value attribute of StopIteration if it was not passed to the constructor.

>>> exc = StopIteration()
>>> exc.value = 42
>>> exc.__reduce__()
(<class 'StopIteration'>, (), {})

Hey, can I work on this?

I get different output for Serhiy's first example now, but the same for the second:

>>> try: import foo
... except Exception as ex: exc = ex
... 
>>> exc.name
'foo'
>>> exc.__reduce__()
(<class 'ModuleNotFoundError'>, ("No module named 'foo'",), {'name': 'foo'})
>>> exc = StopIteration()
>>> exc.value = 42
>>> exc.__reduce__()
(<class 'StopIteration'>, ())
>>>

See also bpo-43460, bpo-32696, bpo-30005.

Attributes __context__, __cause__ and __traceback__ are not pickled. The traceback objects are even not pickleable.

It's not guaranteed that the __cause__ and __context__ are picklable either.

Should we try to pickle them, or should we document this and suggest traceback.TracebackException for storing or transmitting exception information?

Some stuff may be lost in time here, though for my benefit: Why don't the fields on exceptions go into __dict__? If they were there wouldn't serialization work closer-to 'for free'?

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pickle does not serialize Exception cause field #73652

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pickle does not serialize Exception __cause__ field #73652

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diekhans commented on Feb 6, 2017

serhiy-storchaka commented on Feb 8, 2017

kakshay21 commented on Feb 16, 2019

iritkatriel commented on Jun 27, 2021

iritkatriel commented on Jun 27, 2021

iritkatriel commented on Jan 6, 2022

csm10495 commented on Apr 7, 2023

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pickle does not serialize Exception cause field #73652