Add open_dataarray to BackendEntrypoint?

[weiji14]

Is your feature request related to a problem?

The BackendEntrypoint class at

xarray/xarray/backends/common.py

Lines 660 to 738 in 1424972

	class BackendEntrypoint:
	"""
	``BackendEntrypoint`` is a class container and it is the main interface
	for the backend plugins, see :ref:`RST backend_entrypoint`.
	It shall implement:
	- ``open_dataset`` method: it shall implement reading from file, variables
	decoding and it returns an instance of :py:class:`~xarray.Dataset`.
	It shall take in input at least ``filename_or_obj`` argument and
	``drop_variables`` keyword argument.
	For more details see :ref:`RST open_dataset`.
	- ``guess_can_open`` method: it shall return ``True`` if the backend is able to open
	``filename_or_obj``, ``False`` otherwise. The implementation of this
	method is not mandatory.
	- ``open_datatree`` method: it shall implement reading from file, variables
	decoding and it returns an instance of :py:class:`~datatree.DataTree`.
	It shall take in input at least ``filename_or_obj`` argument. The
	implementation of this method is not mandatory. For more details see
	<reference to open_datatree documentation>.
	Attributes
	----------
	open_dataset_parameters : tuple, default: None
	A list of ``open_dataset`` method parameters.
	The setting of this attribute is not mandatory.
	description : str, default: ""
	A short string describing the engine.
	The setting of this attribute is not mandatory.
	url : str, default: ""
	A string with the URL to the backend's documentation.
	The setting of this attribute is not mandatory.
	"""
	open_dataset_parameters: ClassVar[tuple | None] = None
	description: ClassVar[str] = ""
	url: ClassVar[str] = ""
	def __repr__(self) -> str:
	txt = f"<{type(self).__name__}>"
	if self.description:
	txt += f"\n {self.description}"
	if self.url:
	txt += f"\n Learn more at {self.url}"
	return txt
	def open_dataset(
	self,
	filename_or_obj: str | os.PathLike[Any] | ReadBuffer | AbstractDataStore,
	*,
	drop_variables: str | Iterable[str] | None = None,
	) -> Dataset:
	"""
	Backend open_dataset method used by Xarray in :py:func:`~xarray.open_dataset`.
	"""
	raise NotImplementedError()
	def guess_can_open(
	self,
	filename_or_obj: str | os.PathLike[Any] | ReadBuffer | AbstractDataStore,
	) -> bool:
	"""
	Backend open_dataset method used by Xarray in :py:func:`~xarray.open_dataset`.
	"""
	return False
	def open_datatree(
	self,
	filename_or_obj: str | os.PathLike[Any] | ReadBuffer | AbstractDataStore,
	*,
	drop_variables: str | Iterable[str] | None = None,
	) -> DataTree:
	"""
	Backend open_datatree method used by Xarray in :py:func:`~xarray.open_datatree`.
	"""
	raise NotImplementedError()

currently has open_dataset and open_datatree methods, but not open_dataarray. Currently, someone implementing a BackendEntrypoint to do xarray.open_dataarray(..., engine="<something>") would have to implement open_dataset, and behind the scenes, this could result in a roundtrip xr.DataArray -> xr.Dataset -> xr.DataArray conversion, which can lead to some attributes/state getting lost along the way (xref #3268).

Describe the solution you'd like

Enable open_dataarray to be implemented directly, without a roundtrip conversion to/from xr.Dataset

Describe alternatives you've considered

No response

Additional context

GenericMappingTools/pygmt#4008 (comment)