gh-154002: Do not wrap a constructor TypeError in pickle._Unpickler - #154003
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serhiy-storchaka merged 3 commits intoAug 16, 2026
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…iate When a class constructor raised TypeError during old-style (INST/OBJ) unpickling, the pure-Python unpickler did `raise TypeError(msg, err.__traceback__)`, which stored the traceback object in the exception's args and performed no real chaining (__cause__ stayed None). Use `raise TypeError(msg) from err` instead. The C implementation is unaffected; it lets the original error propagate.
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@zware part of EuroPython sprint. |
The prefix repeats the class name that the constructor TypeError already carries, and the C unpickler never wrapped the error, so letting the original propagate removes the traceback-in-args bug and the divergence at once.
_Unpickler._instantiate
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_instantiatewrapped a constructorTypeErrorin a new one, passing the traceback as a second argument, so the traceback object ended up inargsand there was no chaining.The wrapper is now gone and the original error propagates. Its purpose (743d17e, 1998) was to name the class when
__getinitargs__returned something bogus, and the message already carries the class name in that case, so the prefix repeated it:The C unpickler never wrapped the error, so this also removes the divergence between the two implementations. Both now give the same result for a bad argument count and for a
TypeErrorraised inside the constructor body.The test asserts the original error reaches the caller unchanged. It fails on the pure-Python unpickler without this change.
For the record, the traceback-in-
argspart is not from gh-102799 as the issue says. It comes from 26d95c3, whereraise T, V, tbbecameraise T(V, tb)rather thanraise T(V).with_traceback(tb). gh-102799 only changedsys.exc_info()[2]toerr.__traceback__._Unpicklermangles a constructorTypeError#154002