Dataclasses erroring when descriptor has mutable default
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@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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