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gh-145369: Warn against reusing dataclass field objects #145373

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21 changes: 21 additions & 0 deletions Doc/library/dataclasses.rst
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Expand Up @@ -330,6 +330,27 @@ Module contents
:attr:`!C.t` will be ``20``, and the class attributes :attr:`!C.x` and
:attr:`!C.y` will not be set.

.. warning::

Do not reuse :class:`Field` objects across multiple fields or classes.
Each call to :func:`!field` returns a new object that gets modified
in place by the :deco:`dataclass` decorator. Reusing the same field
object will cause unexpected behavior::

f = field(kw_only=True)

@dataclass
class A:
x: int = f # Don't do this
y: int = f # f is now modified and broken

Instead, call :func:`!field` separately for each field::

@dataclass
class A:
x: int = field(kw_only=True)
y: int = field(kw_only=True)

.. versionchanged:: next
If *metadata* is ``None``, use an empty :class:`frozendict`, instead
of a :func:`~types.MappingProxyType` of an empty :class:`dict`.
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