[dataclasses] [typing] Dataclass Protocol

[randolf-scholz]

Feature or enhancement

Currently, the dataclasses module only provides the dataclasses.is_dataclass function to check whether a class is in fact a dataclass.

cpython/Lib/dataclasses.py

Lines 1258 to 1267 in 0a7936a

	def _is_dataclass_instance(obj):
	"""Returns True if obj is an instance of a dataclass."""
	return hasattr(type(obj), _FIELDS)
	def is_dataclass(obj):
	"""Returns True if obj is a dataclass or an instance of a
	dataclass."""
	cls = obj if isinstance(obj, type) else type(obj)
	return hasattr(cls, _FIELDS)

Pitch

Create a Dataclass(Protocol). Classes satisfying the Protocol should be compatible with dataclass methods such as dataclasses.astuple, dataclasses.asdict, dataclasses.fields and dataclasses.replace

It seems that currently the following is possible sufficient:

@runtime_checkable
class Dataclass(Protocol):
    r"""Protocol for dataclasses."""

    @property
    def __dataclass_fields__(self) -> Mapping[str, Field]:
        r"""Return the fields of the dataclass."""

Remarks:

• Additionally one might want to add a MutableDataclass, with __setattr__ and __delattr__.
• As a side benefit, the dataclasses.is_dataclass can be equipped with a Typeguard for Dataclass-Protocol (issubclass(cls, Dataclass) currently won't work because only methods are checked)

[AlexWaygood]

Closing as a duplicate of #102699, which is a newer issue but has more discussion (and a proof-of-concept PR) attached to it.