8000 missing attribute dataclass copy with slots=True & validate_assignment=True · Issue #11768 · pydantic/pydantic · GitHub
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dpinol opened this issue Apr 17, 2025 · 3 comments · Fixed by #11769
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missing attribute dataclass copy with slots=True & validate_assignment=True #11768

dpinol opened this issue Apr 17, 2025 · 3 comments · Fixed by #11769
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@Viicos
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bug V2 Bug related to Pydantic V2

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@dpinol
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dpinol commented Apr 17, 2025

Initial Checks

  • I confirm that I'm using Pydantic V2

Description

When copying a pydantic v2 dataclass with both "validate_assignment": True and slots=True, I get object has no attribute for an existing field.

Example Code

from copy import copy

from pydantic.dataclasses import dataclass


@dataclass(slots=True, config={"validate_assignment": True})
class Sut:
    b1: int

sut = Sut(0)
print(sut.b1)
copy(sut)

Python, Pydantic & OS Version

pydantic version: 2.11.3
        pydantic-core version: 2.33.1
          pydantic-core build: profile=release pgo=false
                 install path: /home/dani/.virtualenvs/sc-api-DipHMMiG/lib/python3.12/site-packages/pydantic
               python version: 3.12.9 (main, Apr  4 2025, 08:52:04) [GCC 14.2.0]
                     platform: Linux-6.11.0-8-generic-x86_64-with-glibc2.40
             related packages: typing_extensions-4.13.2 mypy-1.15.0
                       commit: unknown
@dpinol dpinol added bug V2 Bug related to Pydantic V2 pending Is unconfirmed labels Apr 17, 2025
@Viicos
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Viicos commented Apr 17, 2025

I'm not sure how to fix this yet. We could define a custom __copy__() method on Pydantic dataclasses, but the issue lies in the fact that pickling (see repro below) uses setattr(), and pydantic-core excepts field values to be present when validating on assignment. Probably we can override __setstate__() but I'll have to check with the stdlib implementation.

Repro with pickling:

import pickle

from pydantic.dataclasses import dataclass


@dataclass(slots=True, config={"validate_assignment": True})
class Sut:
    b1: int

pickle.loads(pickle.dumps(Sut(0)))
# AttributeError

@Viicos Viicos removed the pending Is unconfirmed label Apr 17, 2025
@Viicos Viicos self-assigned this Apr 17, 2025
@Viicos Viicos mentioned this issue Apr 17, 2025
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@dpinol
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dpinol commented May 27, 2025

Thanks for the fix!
Any timeframe for a release?

@Viicos
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Viicos commented May 28, 2025

2.12 is planned on June 30th.

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@Viicos Viicos

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bug V2 Bug related to Pydantic V2
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Make sure Pydantic dataclasses with slots and validate_assignment can be unpickled pydantic/pydantic
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@dpinol @Viicos
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