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Pickling deque subclasses ignores their __dict__. #4769
Description
Caught after updating test_deque.py. In RustPython, pickling a deque subclass ignores its __dict__:
>>>>> import pickle
>>>>> from collections import deque
>>>>> class Deque(deque): pass
>>>>> d = Deque()
>>>>> d.x = "value"
>>>>> e = pickle.loads(pickle.dumps(pickle.HIGHEST_PROTOCOL))
>>>>> e.__dict__
{}While in Python, __dict__ is correctly returned as {'x': 'value'}.
This doesn't appear to be the case with some other collections like list and dict so those could probably serve as an inspiration for what might be missing in deque.
For testing purposes, see test_copy_pickle in Lib/test/test_deque.py.
Activity
youknowone commentedon Mar 25, 2023
Probably we are ignoring subclasses
__dict__for most of our__reduce__implementations.DimitrisJim commentedon Mar 25, 2023
Yeah, I'd bet
collectionsshould have most of them. Haven't checked the tests but ifdequeshowed up now maybe similar ones will appear for the rest of the collections soon.youknowone commentedon Oct 6, 2026
The originally reported problem no longer reproduces: pickling a deque subclass preserves instance
__dict__(test_copy_picklepasses). Closing per #8982.A leftover
expectedFailureremains ontest_pickle_recursiveforDequeWithBadIter(pickle does not raiseTypeErrorwhen__iter__fails). That is a separate edge case from dropping subclass__dict__.— commented by Grok 4.6