pickle exceptions with mandatory keyword args will traceback

[abadger]

BPO	40917
Nosy	@abadger, @remilapeyre

^{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 2020-06-12.02:00:02.122>
created_at = <Date 2020-06-08.23:46:10.416>
labels = ['3.8', '3.7', 'library', '3.9', '3.10']
title = 'pickle exceptions with mandatory keyword args will traceback'
updated_at = <Date 2020-06-12.02:00:02.120>
user = 'https://github.com/abadger'

bugs.python.org fields:

activity = <Date 2020-06-12.02:00:02.120>
actor = 'a.badger'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-12.02:00:02.122>
closer = 'a.badger'
components = ['Library (Lib)']
creation = <Date 2020-06-08.23:46:10.416>
creator = 'a.badger'
dependencies = []
files = []
hgrepos = []
issue_num = 40917
keywords = []
message_count = 3.0
messages = ['371057', '371075', '371328']
nosy_count = 2.0
nosy_names = ['a.badger', 'remi.lapeyre']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue40917'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[abadger]

I was trying to use multiprocessing (via a concurrent.futures.ProcessPoolExecutor) and encountered an error when pickling a custom Exception. On closer examination I was able to create a simple test case that only involves pickle:

import pickle
class StrRegexError(Exception):
    def __init__(self, *, pattern):
        self.pattern = pattern

data = pickle.dumps(StrRegexError(pattern='test'))
instance = pickle.loads(data)

[pts/11@peru /srv/ansible]$ python3.8 ~/p.py
Traceback (most recent call last):
  File "/home/badger/p.py", line 7, in <module>
    instance = pickle.loads(data)
TypeError: __init__() missing 1 required keyword-only argument: 'pattern'

pickle can handle mandatory keyword args in other classes derived from object; it's only classes derived from Exception that have issues.

[remilapeyre]

This is a duplicate of bpo-27015.

[abadger]

Thanks! I confirm that your PR #11580 for https://bugs.python.org/issue27015 fixes this problem.

Closing this one.