Generate __reduce__ for auto_exc classes so kw_only exceptions pickle#1595
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Jayashanker-Padishala wants to merge 2 commits into
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Generate __reduce__ for auto_exc classes so kw_only exceptions pickle#1595Jayashanker-Padishala wants to merge 2 commits into
Jayashanker-Padishala wants to merge 2 commits into
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BaseException.__reduce__ re-creates an exception by calling cls(*self.args), which fails when __init__ cannot accept the args positionally (e.g. kw_only fields) and loses state __init__ does not reproduce. Generate a dedicated __reduce__ for exceptions that re-creates the instance with BaseException.__new__ (setting args without calling __init__) and restores attribute state: slots classes go through their generated frozen-safe __setstate__, dict classes apply state inside the reconstructor because default state application can go through setattr, which frozen classes reject. Fixes python-attrs#734
ada33-arch
approved these changes
Jul 24, 2026
ada33-arch
approved these changes
Jul 24, 2026
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Fixes #734
Summary
BaseException.__reduce__re-creates an exception by callingcls(*self.args)— that fails when__init__can't accept the args positionally (kw_onlyfields) and silently loses any state__init__doesn't reproduce.Following the direction in the issue (a dedicated
__reduce__when we detect an exception), but addressing @meshy's counterexample —(cls, (), state)breaks for subclasses with required fields becausecls()still calls__init__:BaseException.__new__(cls, *args)via a module-level reconstructor:argsis set without__init__ever running, so signatures don't matter and validators/converters don't re-run on unpickle.__setstate__(frozen-safe viaobject.__setattr__); the generated__reduce__tolerates attributes that were never set (init=False, no default).__dict__state inside the reconstructor, because the default pickle state application can go throughsetattr, which frozen classes reject; this also preserves non-attrs instance attributes, matching classic exception pickling.__reduce__on the class is respected (not overwritten).Tests
New tests in
tests/test_functional.py: kw_only round-trip across dict/slots × plain/frozen and all pickle protocols ≥2; subclass-with-required-positional round-trip; unset-attribute (init=False) pickling + deepcopy; user__reduce__respected. Full suite: 1412 passed.Changelog fragment included (named
1567.change.md— happy to rename to the actual PR number).