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dataclass(slots=True) fails if method has an empty __class__ cell #148947

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dataclass(slots=True) fails if method has an empty __class__ cell#148947
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JelleZijlstra
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3.14bugs and security fixes3.15bugs and security fixesstdlibStandard Library Python modules in the Lib/ directorytopic-dataclassestype-bugAn unexpected behavior, bug, or error

Description

@JelleZijlstra
JelleZijlstra
opened on Apr 24, 2026

Bug report

Bug description:

On Python 3.14, this code:

def maker():
    if False:
        __class__ = 42

    def method(self):
        return __class__
    return method

from dataclasses import dataclass

@dataclass(slots=True)
class X:
    a: int

    meth = maker()

Fails with ValueError: Cell is empty, because some dataclasses machinery for updating the __class__ cell doesn't reckon with the possibility that the cell may be empty.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

macOS

Linked PRs

Activity

  1. JelleZijlstra
    added
    type-bugAn unexpected behavior, bug, or error
    on Apr 24, 2026
  2. JelleZijlstra
    self-assigned this
    on Apr 24, 2026
  3. johnslavik
    changed the title [-]dataclass(slots=True) crashes if method has an empty `__class__` cell[/-] [+]dataclass(slots=True) fails if method has an empty `__class__` cell[/+] on Apr 24, 2026
  4. johnslavik
    changed the title [-]dataclass(slots=True) fails if method has an empty `__class__` cell[/-] [+]dataclass(slots=True) crashes if method has an empty `__class__` cell[/+] on Apr 24, 2026
  5. picnixz
    added
    stdlibStandard Library Python modules in the Lib/ directory
    topic-dataclasses
    on Apr 24, 2026
  6. johnslavik
    added
    3.14bugs and security fixes
    3.15bugs and security fixes
    on Apr 24, 2026
  7. johnslavik
    changed the title [-]dataclass(slots=True) crashes if method has an empty `__class__` cell[/-] [+]dataclass(slots=True) fails if method has an empty `__class__` cell[/+] on Apr 24, 2026
  8. JelleZijlstra
    added a commit that references this issue on Apr 25, 2026

    gh-148947: dataclasses: fix error on empty __class__ cell (#148948)

    6d7bbee
  9. ljfp
    added a commit that references this issue on Apr 25, 2026

    pythongh-148947: dataclasses: fix error on empty __class__ cell (pyth…

    efe54da
  10. JelleZijlstra
    added a commit that references this issue on Apr 25, 2026

    [3.14] gh-148947: dataclasses: fix error on empty __class__ cell (GH-…

    15e2b64
  11. JelleZijlstra
    closed this as completedon Apr 25, 2026
  12. gpshead
    added 3 commits that reference this issue on Apr 25, 2026

    [3.13] pythongh-148947: dataclasses: fix error on empty __class__ cell (

    3abfebc

    [3.13] pythongh-148947: dataclasses: fix error on empty __class__ cell (

    678cc46

    [3.13] gh-105936: Properly update closure cells for __setattr__ and _…

    9651429
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3.14bugs and security fixes3.15bugs and security fixesstdlibStandard Library Python modules in the Lib/ directorytopic-dataclassestype-bugAn unexpected behavior, bug, or error

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