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Undocumented behavior change in pickle: PicklingError raised earlier for local objects in Python 3.14 #139806
Description
Summary
pickle has changed from raising an AttributeError to PicklingError for functions containing locals in their reduced path. This change is not reflected in the what's new in 3.14 for pickle.
Example
>>> import pickle
>>>
>>> def some_function(a, b):
... def some_inner_function(c, d):
... return a + b + c + d
... pickle.dumps(some_inner_function)
>>>
>>> try:
... some_function(1, 2)
... except AttributeError as e:
... print(e)
Can't pickle local object 'some_function.<locals>.some_inner_function'on 3.14 this raises _pickle.PicklingError: Can't pickle local object <function some_function.<locals>.some_inner_function at 0x7fb4fd1a3060>
Cf. the CI in this repo for tests on 3.11-3.14.
Impact
Code that expected and caught AttributeError now gets PicklingError
Python versions
Works in 3.11-3.13, changed in 3.14
Resolution
A comment in the changelog, or a reversion to raising an AttributeError
Activity
StanFromIreland commentedon Oct 8, 2025
Seems like a result of b2a8c38, cc @serhiy-storchaka
serhiy-storchaka commentedon Oct 8, 2025
The Python implementation already raised PicklingError in this example. #122311 unified and improved pickle errors. You can find it in the full changelog, but I think that this change is not significant to be added in What's New.
picnixz commentedon Oct 9, 2025
Considering it's part of the full changelog, I also believe it's fine not to document it in What's New. As a user, I would actually expect PicklingError instead of AttributeError. However, we could add it in "Porting to Python 3.14" as it appears users were catching those exceptions. The docs say are quite vague though, saying that we can many different exception types during unpickling but not during pickling (omission? deliberate?)
gh-139806: Mention pickle error changes in What's New in 3.14 (GH-154020
[3.15] gh-139806: Mention pickle error changes in What's New in 3.14 (G…
[3.14] gh-139806: Mention pickle error changes in What's New in 3.14 (G…
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