Description

JelleZijlstra

opened on Apr 24, 2026

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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

• gh-148947: dataclasses: fix error on empty __class__ cell  #148948
• [3.14] gh-148947: dataclasses: fix error on empty __class__ cell (GH-148948) #148995

Activity

1. JelleZijlstra

   added

   type-bugAn unexpected behavior, bug, or errorAn 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/ directoryStandard Library Python modules in the Lib/ directory

   topic-dataclasses

   on Apr 24, 2026
6. johnslavik

   added

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs 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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