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## Summary
The previous documentation sounded as if typing a mutable default as a
`ClassVar` were optional. However, it is not, as not doing so causes a
`ValueError`. The snippet below was tested in Python's interactive
shell:
```
>>> from dataclasses import dataclass
>>> @dataclass
... class A:
... mutable_default: list[int] = []
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.11/dataclasses.py", line 1230, in dataclass
return wrap(cls)
^^^^^^^^^
File "/usr/lib/python3.11/dataclasses.py", line 1220, in wrap
return _process_class(cls, init, repr, eq, order, unsafe_hash,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/dataclasses.py", line 958, in _process_class
cls_fields.append(_get_field(cls, name, type, kw_only))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/dataclasses.py", line 815, in _get_field
raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class 'list'> for field mutable_default is not allowed: use default_factory
>>>
```
This behavior is also documented in Python's docs, see
[here](https://docs.python.org/3/library/dataclasses.html#mutable-default-values):
> [...] the
[dataclass()](https://docs.python.org/3/library/dataclasses.html#dataclasses.dataclass)
decorator will raise a
[ValueError](https://docs.python.org/3/library/exceptions.html#ValueError)
if it detects an unhashable default parameter. The assumption is that if
a value is unhashable, it is mutable. This is a partial solution, but it
does protect against many common errors.
And
[here](https://docs.python.org/3/library/dataclasses.html#class-variables)
it is documented why it works if it is typed as a `ClassVar`:
> One of the few places where
[dataclass()](https://docs.python.org/3/library/dataclasses.html#dataclasses.dataclass)
actually inspects the type of a field is to determine if a field is a
class variable as defined in [PEP
526](https://peps.python.org/pep-0526/). It does this by checking if the
type of the field is typing.ClassVar. If a field is a ClassVar, it is
excluded from consideration as a field and is ignored by the dataclass
mechanisms. Such ClassVar pseudo-fields are not returned by the
module-level
[fields()](https://docs.python.org/3/library/dataclasses.html#dataclasses.fields)
function.
In this PR I have changed the documentation to make it a little bit
clearer that not using `ClassVar` makes the code invalid.
|
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|---|---|---|
| .. | ||
| ruff | ||
| ruff_benchmark | ||
| ruff_cache | ||
| ruff_dev | ||
| ruff_diagnostics | ||
| ruff_formatter | ||
| ruff_index | ||
| ruff_linter | ||
| ruff_macros | ||
| ruff_notebook | ||
| ruff_python_ast | ||
| ruff_python_codegen | ||
| ruff_python_formatter | ||
| ruff_python_index | ||
| ruff_python_literal | ||
| ruff_python_parser | ||
| ruff_python_resolver | ||
| ruff_python_semantic | ||
| ruff_python_stdlib | ||
| ruff_python_trivia | ||
| ruff_server | ||
| ruff_shrinking | ||
| ruff_source_file | ||
| ruff_text_size | ||
| ruff_wasm | ||
| ruff_workspace | ||