Pickling and copying exceptions doesn't preserve non-__dict__ attributes

[serhiy-storchaka]

BPO	30005
Nosy	@avassalotti, @serhiy-storchaka, @iritkatriel
Dependencies	bpo-29998: Pickling and copying ImportError doesn't preserve name and path

^{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 2017-04-06.05:59:35.082>
labels = ['interpreter-core', 'type-bug', '3.11']
title = "Pickling and copying exceptions doesn't preserve non-__dict__ attributes"
updated_at = <Date 2021-06-27.21:37:57.332>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-06-27.21:37:57.332>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2017-04-06.05:59:35.082>
creator = 'serhiy.storchaka'
dependencies = ['29998']
files = []
hgrepos = []
issue_num = 30005
keywords = []
message_count = 2.0
messages = ['291212', '396602']
nosy_count = 3.0
nosy_names = ['alexandre.vassalotti', 'serhiy.storchaka', 'iritkatriel']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30005'
versions = ['Python 3.11']

[serhiy-storchaka]

Pickling and copying exceptions preserves only __dict__ attributes.

This includes writeable internal fields initialized in constructor:

>>> import pickle, copy
>>> e = StopIteration(12)
>>> e.value = 34
>>> e.value
34
>>> e2 = pickle.loads(pickle.dumps(e, 4))
>>> e2.value
12
>>> e2 = copy.copy(e)
>>> e2.value
12

And __slots__:

>>> class E(Exception): __slots__ = ('x', 'y')
... 
>>> e = E()
>>> e.x = 12
>>> e.x
12
>>> e2 = pickle.loads(pickle.dumps(e, 4))
>>> e2.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x
>>> e2 = copy.copy(e)
>>> e2.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x

__context__, __cause__ and __traceback__ are lost too (see bpo-29466).

bpo-26579 is similar, but resolving it will not resolve this issue since BaseException has its own __reduce__ and __setstate__ implementations.

The solution of this issue will look similar to bpo-29998, but more complex and general.

[iritkatriel]

See also bpo-43460, bpo-32696, bpo-29466.