python / python/cpython

Dataclasses erroring when descriptor has mutable default

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#132,559 10 comments 0 reactions 1 assignee View on GitHub

@ericvsmith is already working on this.

Since Apr 15, 2025.

stdlib topic-dataclasses type-bug
Dominant language
Python
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Description

Bug report

Bug description:

Running the following code:

from dataclasses import dataclass

class Descriptor:
    def __set_name__(self, owner, name: str) -> None:
        self.name = name

    def __get__(self, instance, owner) -> list[str]:
        if instance is None:
            return []
        return instance.__dict__[self.name]

    def __set__(self, instance, value: list[str]) -> None:
        instance.__dict__[self.name] = value


@dataclass()
class TestFoo:
    apple: Descriptor = Descriptor()

print(TestFoo().apple)

results in:

  File "/opt/homebrew/Cellar/python@3.13/3.13.3/Frameworks/Python.framework/Versions/3.13/lib/python3.13/dataclasses.py", line 860, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
                     f'{f.name} is not allowed: use default_factory')
ValueError: mutable default <class 'list'> for field apple is not allowed: use default_factory
CPython versions tested on:

3.13

Operating systems tested on:

macOS

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