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Pickling deque subclasses ignores their __dict__. #4769

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Pickling deque subclasses ignores their __dict__.#4769
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C-bugSomething isn't workingC-compatA discrepancy between RustPython and CPython

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@DimitrisJim
DimitrisJim
opened on Mar 25, 2023

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.

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  1. DimitrisJim
    added
    C-bugSomething isn't working
    C-compatA discrepancy between RustPython and CPython
    on Mar 25, 2023

  2. youknowone commented on Mar 25, 2023

    @youknowone
    youknowone
    on Mar 25, 2023
    Member

    Probably we are ignoring subclasses __dict__ for most of our __reduce__ implementations.

  3. DimitrisJim commented on Mar 25, 2023

    @DimitrisJim
    DimitrisJim
    on Mar 25, 2023
    MemberAuthor

    Yeah, I'd bet collections should have most of them. Haven't checked the tests but if deque showed up now maybe similar ones will appear for the rest of the collections soon.

  4. youknowone commented on Oct 6, 2026

    @youknowone
    youknowone
    on Oct 6, 2026
    Member

    The originally reported problem no longer reproduces: pickling a deque subclass preserves instance __dict__ (test_copy_pickle passes). Closing per #8982.

    A leftover expectedFailure remains on test_pickle_recursive for DequeWithBadIter (pickle does not raise TypeError when __iter__ fails). That is a separate edge case from dropping subclass __dict__.

    — commented by Grok 4.6

  5. youknowone
    closed this as completedon Oct 6, 2026
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