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pickle: pure-Python _Unpickler mangles a constructor TypeError #154002
Description
The pure-Python pickle._Unpickler mishandles a constructor failure while unpickling old-style instances (the INST and OBJ opcodes). If the class constructor raises TypeError, _instantiate does:
raise TypeError("in constructor for %s: %s" %
(klass.__name__, str(err)), err.__traceback__)Passing err.__traceback__ as a second positional argument puts the traceback object into the exception's args, so it leaks into str(), and there is no real chaining (__cause__ stays None).
import pickle, io
class Bad:
def __init__(self, *a):
raise TypeError("boom")
u = pickle._Unpickler(io.BytesIO())
try:
u._instantiate(Bad, (1,))
except TypeError as e:
print(e.args) # ('in constructor for Bad: boom', <traceback object ...>)
print(e.__cause__) # NoneThe C _pickle unpickler does not wrap the error at all, it just lets the original TypeError propagate, so this only affects the pure-Python implementation.
The fix is to drop the wrapper and let the original TypeError propagate:
value = klass(*args)The wrapper was added in 743d17e (1998) to name the class when __getinitargs__ returned something bogus. The constructor error already names the class in that case, so the prefix only repeats it:
in constructor for WrongArgCount: WrongArgCount.__init__() takes 2 positional arguments but 3 were given
The traceback in args is a leftover from the Python 2 way of carrying one. It comes from 26d95c3, where raise T, V, tb became raise T(V, tb) instead of raise T(V).with_traceback(tb). gh-102799 later changed sys.exc_info()[2] to err.__traceback__ and nothing else.
Found while going through devdanzin's audit of the standard library, item 15: https://gist.github.com/devdanzin/3198710e3c0128fda5e0a7b4e0768e5f
Activity
Merge branch 'main' into fix-pythongh-154002-pickle-instantiate-chaining
serhiy-storchaka commentedon Jul 30, 2026
The
err.__traceback__-in-argsleak is real, but before fixing it with... from err, it's worth asking whether the wrapper should stay at all.Its purpose (from Guido, 1998 — 743d17e) was to add the class name to a constructor
TypeError. That's now redundant: a bad__getinitargs__— the case it targeted — already producesOldStyleBogus.__init__() takes 2 positional arguments but 3 were given. The prefix just repeats a name that's already there. The only case it adds anything is aTypeErrorraised inside the constructor body, where the class is still visible in the traceback anyway.So the simplest fix may be to drop the wrapper and let the original
TypeErrorpropagate. That also matches the C_pickleunpickler, which never wrapped it — closing the Python/C divergence instead of only fixing the Python side.(Side note: the tb-in-
argsbug isn't from gh-102799; it dates to the Py3 conversion 26d95c3, whereraise T, V, tbbecameraise T(V, tb)instead ofraise T(V).with_traceback(tb). gh-102799 only renamed the tb expression.)[-]pickle: pure-Python `_Unpickler._instantiate` mangles a constructor TypeError (traceback in args, no chaining)[/-][+]pickle: pure-Python `_Unpickler` mangles a constructor `TypeError`[/+]fedonman commentedon Aug 5, 2026
@serhiy-storchaka Good call, dropped the wrapper.
You are right that the prefix is redundant. For the
__getinitargs__case it actually repeats the name twice:Both unpicklers now give the same error for a bad argument count and for a
TypeErrorraised in the constructor body, so the divergence is gone too.You are also right about the provenance. It comes from 26d95c3, where
raise T, V, tbbecameraise T(V, tb)instead ofraise T(V).with_traceback(tb). gh-102799 only changedsys.exc_info()[2]toerr.__traceback__.GH-154003 is updated and ready for review.
gh-154002: Do not wrap a constructor TypeError in pickle._Unpickler (G…