Description

drhagen

opened on May 1, 2023

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You are supposed to be able to override how pickle pickles an object with __getstate__ and __setstate__. However, these methods are ignored in a dataclass when specifying both frozen=True and slots=True.

See this example:

import pickle
from dataclasses import dataclass


@dataclass(frozen=True, slots=True)
class Foo:
    bar: int

    def __getstate__(self):
        print("getstate")
        return {"bar": self.bar}

    def __setstate__(self, state):
        print("setstate")
        object.__setattr__(self, "bar", state["bar"])

b = pickle.dumps(Foo(1))
foo = pickle.loads(b)

The expected "getstate" and "setstate" lines are never printed because the supplied methods are never called. If either frozen or slots is removed, the expected lines are printed.

From the source code, it is pretty clear why this is happening. If frozen and slots are both True, then special versions of __getstate__ and __setstate__ are unconditionally attached to the dataclass. The dataclass decorator should probably treat this way that it does other methods—only add a method if it is not already present.

Linked PRs

• gh-104035: Do not ignore user-defined __{get,set}state__ in slotted frozen dataclasses #104041
• [3.11] gh-104035: Do not ignore user-defined __{get,set}state__ in slotted frozen dataclasses (GH-104041) #104044

Activity

1. drhagen

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on May 1, 2023
2. ericvsmith commented on May 1, 2023

   

   ericvsmith

   on May 1, 2023

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   It does seem reasonable to ignore __getstate__ and __setstate__ if they're already set. Have you tested this? And in particular, does it break any existing tests?

   Reacted by David Hagen
3. AlexWaygood

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on May 1, 2023
4. sobolevn

   added a commit that references this issue on May 1, 2023

   pythongh-104035: Do not ignore user-defined `__{get,set}state__` in s…

   b6a09a4
5. sobolevn commented on May 1, 2023

   

   sobolevn

   on May 1, 2023

   Member

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   Oh, I just went for fixing this in #104041 :)

   New logic will allow users to customize __getstate__ and __setstate__ method.
   Thanks a lot, @drhagen for finding this :)

   It is funny that I've spent the whole day optimizing these two in #103032, but I haven't seen this! 👍

   Reacted by David Hagen
6. carljm

   added a commit that references this issue on May 1, 2023

   gh-104035: Do not ignore user-defined `__{get,set}state__` in slotted…

   99aab61
7. miss-islington

   added a commit that references this issue on May 1, 2023

   pythongh-104035: Do not ignore user-defined `__{get,set}state__` in s…

   9141fed
8. carljm

   linked a pull request that will close this issue[3.11] gh-104035: Do not ignore user-defined `__{get,set}state__` in slotted frozen dataclasses (GH-104041) #104044on May 1, 2023
9. carljm

   added a commit that references this issue on May 1, 2023

   [3.11] gh-104035: Do not ignore user-defined `__{get,set}state__` in …

   0df7c3a
10. carljm

    closed this as completedon May 1, 2023
11. carljm

    added a commit that references this issue on May 1, 2023

    Merge branch 'main' into inlinecomp2

    1274e2b

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