Add Coordinates.from_xindex method (+ refactor API doc) (#10000)

* add Coordinates.from_xindex method

* doc: refactor Coordinates API reference

Make it more consistent with ``Dataset``, ``DataArray`` and
``DataTree``. The ``Coordinates`` class is 2nd order compared to the
former ones, but it is public API and useful (for creating coordinates
from indexes and merging coordinates together) so it deserves its own
(expanded) section + summary tables in the API reference doc.

* add tests

* update what's new

* fix doc build?

* docstring tweaks

* doc (api): add missing Coordinates.sizes property

* update what's new (documentation)

* improve docstrings on Coordinates creation

* doc: move what's new entries after last release

---------

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

doc/api-hidden.rst

@@ -9,30 +9,12 @@
 .. autosummary::
    :toctree: generated/
 
-   Coordinates.from_pandas_multiindex
-   Coordinates.get
-   Coordinates.items
-   Coordinates.keys
-   Coordinates.values
-   Coordinates.dims
-   Coordinates.dtypes
-   Coordinates.variables
-   Coordinates.xindexes
-   Coordinates.indexes
-   Coordinates.to_dataset
-   Coordinates.to_index
-   Coordinates.update
-   Coordinates.assign
-   Coordinates.merge
-   Coordinates.copy
-   Coordinates.equals
-   Coordinates.identical
-
    core.coordinates.DatasetCoordinates.get
    core.coordinates.DatasetCoordinates.items
    core.coordinates.DatasetCoordinates.keys
    core.coordinates.DatasetCoordinates.values
    core.coordinates.DatasetCoordinates.dims
+   core.coordinates.DatasetCoordinates.sizes
    core.coordinates.DatasetCoordinates.dtypes
    core.coordinates.DatasetCoordinates.variables
    core.coordinates.DatasetCoordinates.xindexes
@@ -74,6 +56,7 @@
    core.coordinates.DataArrayCoordinates.keys
    core.coordinates.DataArrayCoordinates.values
    core.coordinates.DataArrayCoordinates.dims
+   core.coordinates.DataArrayCoordinates.sizes
    core.coordinates.DataArrayCoordinates.dtypes
    core.coordinates.DataArrayCoordinates.variables
    core.coordinates.DataArrayCoordinates.xindexes
@@ -104,6 +87,25 @@
    core.weighted.DataArrayWeighted.obj
    core.weighted.DataArrayWeighted.weights
 
+   core.coordinates.DataTreeCoordinates.get
+   core.coordinates.DataTreeCoordinates.items
+   core.coordinates.DataTreeCoordinates.keys
+   core.coordinates.DataTreeCoordinates.values
+   core.coordinates.DataTreeCoordinates.dims
+   core.coordinates.DataTreeCoordinates.sizes
+   core.coordinates.DataTreeCoordinates.dtypes
+   core.coordinates.DataTreeCoordinates.variables
+   core.coordinates.DataTreeCoordinates.xindexes
+   core.coordinates.DataTreeCoordinates.indexes
+   core.coordinates.DataTreeCoordinates.to_dataset
+   core.coordinates.DataTreeCoordinates.to_index
+   core.coordinates.DataTreeCoordinates.update
+   core.coordinates.DataTreeCoordinates.assign
+   core.coordinates.DataTreeCoordinates.merge
+   core.coordinates.DataTreeCoordinates.copy
+   core.coordinates.DataTreeCoordinates.equals
+   core.coordinates.DataTreeCoordinates.identical
+
    core.accessor_dt.DatetimeAccessor.ceil
    core.accessor_dt.DatetimeAccessor.floor
    core.accessor_dt.DatetimeAccessor.round

doc/api.rst

@@ -67,6 +67,7 @@ Attributes
    Dataset.attrs
    Dataset.encoding
    Dataset.indexes
+   Dataset.xindexes
    Dataset.chunks
    Dataset.chunksizes
    Dataset.nbytes
@@ -275,6 +276,7 @@ Attributes
    DataArray.attrs
    DataArray.encoding
    DataArray.indexes
+   DataArray.xindexes
    DataArray.chunksizes
 
 ndarray attributes
@@ -897,6 +899,84 @@ Methods copied from :py:class:`numpy.ndarray` objects, here applying to the data
 ..    DataTree.sortby
 ..    DataTree.broadcast_like
 
+Coordinates
+===========
+
+Creating coordinates
+--------------------
+
+.. autosummary::
+   :toctree: generated/
+
+   Coordinates
+   Coordinates.from_xindex
+   Coordinates.from_pandas_multiindex
+
+Attributes
+----------
+
+.. autosummary::
+   :toctree: generated/
+
+   Coordinates.dims
+   Coordinates.sizes
+   Coordinates.dtypes
+   Coordinates.variables
+   Coordinates.indexes
+   Coordinates.xindexes
+
+Dictionary Interface
+--------------------
+
+Coordinates implement the mapping interface with keys given by variable names
+and values given by ``DataArray`` objects.
+
+.. autosummary::
+   :toctree: generated/
+
+   Coordinates.__getitem__
+   Coordinates.__setitem__
+   Coordinates.__delitem__
+   Coordinates.update
+   Coordinates.get
+   Coordinates.items
+   Coordinates.keys
+   Coordinates.values
+
+Coordinates contents
+--------------------
+
+.. autosummary::
+   :toctree: generated/
+
+   Coordinates.to_dataset
+   Coordinates.to_index
+   Coordinates.assign
+   Coordinates.merge
+   Coordinates.copy
+
+Comparisons
+-----------
+
+.. autosummary::
+   :toctree: generated/
+
+   Coordinates.equals
+   Coordinates.identical
+
+Proxies
+-------
+
+Coordinates that are accessed from the ``coords`` property of Dataset, DataArray
+and DataTree objects, respectively.
+
+.. autosummary::
+   :toctree: generated/
+
+   core.coordinates.DatasetCoordinates
+   core.coordinates.DataArrayCoordinates
+   core.coordinates.DataTreeCoordinates
+
 Universal functions
 ===================
 
@@ -1107,27 +1187,6 @@ Coder objects
 
    coders.CFDatetimeCoder
 
-Coordinates objects
-===================
-
-Dataset
--------
-
-.. autosummary::
-   :toctree: generated/
-
-   core.coordinates.DatasetCoordinates
-   core.coordinates.DatasetCoordinates.dtypes
-
-DataArray
----------
-
-.. autosummary::
-   :toctree: generated/
-
-   core.coordinates.DataArrayCoordinates
-   core.coordinates.DataArrayCoordinates.dtypes
-
 Plotting
 ========
 

doc/whats-new.rst

@@ -21,6 +21,9 @@ v2025.02.0 (unreleased)
 
 New Features
 ~~~~~~~~~~~~
+- Added :py:meth:`Coordinates.from_xindex` as convenience for creating a new :py:class:`Coordinates` object
+  directly from an existing Xarray index object if the latter supports it (:pull:`10000`)
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 - Allow kwargs in :py:meth:`DataTree.map_over_datasets` and :py:func:`map_over_datasets` (:issue:`10009`, :pull:`10012`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 - support python 3.13 (no free-threading) (:issue:`9664`, :pull:`9681`)
@@ -52,7 +55,8 @@ Bug fixes
 
 Documentation
 ~~~~~~~~~~~~~
-
+- Better expose the :py:class:`Coordinates` class in API reference (:pull:`10000`)
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 
 Internal Changes
 ~~~~~~~~~~~~~~~~
@@ -131,6 +135,11 @@ New Features
   :py:class:`pandas.DatetimeIndex` (:pull:`9965`). By `Spencer Clark
   <https://github.com/spencerkclark>`_ and `Kai Mühlbauer
   <https://github.com/kmuehlbauer>`_.
+- :py:meth:`DatasetGroupBy.first` and :py:meth:`DatasetGroupBy.last` can now use ``flox`` if available. (:issue:`9647`)
+  By `Deepak Cherian <https://github.com/dcherian>`_.
+
+Breaking changes
+~~~~~~~~~~~~~~~~
 - Adds shards to the list of valid_encodings in the zarr backend, so that
   sharded Zarr V3s can be written (:issue:`9947`, :pull:`9948`).
   By `Jacob Prince_Bieker <https://github.com/jacobbieker>`_

xarray/core/coordinates.py

@@ -200,10 +200,17 @@ class Coordinates(AbstractCoordinates):
 
     - returned via the :py:attr:`Dataset.coords`, :py:attr:`DataArray.coords`,
       and :py:attr:`DataTree.coords` properties,
-    - built from Pandas or other index objects
-      (e.g., :py:meth:`Coordinates.from_pandas_multiindex`),
-    - built directly from coordinate data and Xarray ``Index`` objects (beware that
-      no consistency check is done on those inputs),
+    - built from Xarray or Pandas index objects
+      (e.g., :py:meth:`Coordinates.from_xindex` or
+      :py:meth:`Coordinates.from_pandas_multiindex`),
+    - built manually from input coordinate data and Xarray ``Index`` objects via
+      :py:meth:`Coordinates.__init__` (beware that no consistency check is done
+      on those inputs).
+
+    To create new coordinates from an existing Xarray ``Index`` object, use
+    :py:meth:`Coordinates.from_xindex` instead of
+    :py:meth:`Coordinates.__init__`. The latter is useful, e.g., for creating
+    coordinates with no default index.
 
     Parameters
     ----------
@@ -352,6 +359,35 @@ def _construct_direct(
         )
         return obj
 
+    @classmethod
+    def from_xindex(cls, index: Index) -> Self:
+        """Create Xarray coordinates from an existing Xarray index.
+
+        Parameters
+        ----------
+        index : Index
+            Xarray index object. The index must support generating new
+            coordinate variables from itself.
+
+        Returns
+        -------
+        coords : Coordinates
+            A collection of Xarray indexed coordinates created from the index.
+
+        """
+        variables = index.create_variables()
+
+        if not variables:
+            raise ValueError(
+                "`Coordinates.from_xindex()` only supports index objects that can generate "
+                "new coordinate variables from scratch. The given index (shown below) did not "
+                f"create any coordinate.\n{index!r}"
+            )
+
+        indexes = {name: index for name in variables}
+
+        return cls(coords=variables, indexes=indexes)
+
     @classmethod
     def from_pandas_multiindex(cls, midx: pd.MultiIndex, dim: Hashable) -> Self:
         """Wrap a pandas multi-index as Xarray coordinates (dimension + levels).

xarray/tests/test_coordinates.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from collections.abc import Mapping
+
 import numpy as np
 import pandas as pd
 import pytest
@@ -8,7 +10,7 @@
 from xarray.core.coordinates import Coordinates
 from xarray.core.dataarray import DataArray
 from xarray.core.dataset import Dataset
-from xarray.core.indexes import PandasIndex, PandasMultiIndex
+from xarray.core.indexes import Index, PandasIndex, PandasMultiIndex
 from xarray.core.variable import IndexVariable, Variable
 from xarray.tests import assert_identical, source_ndarray
 
@@ -73,6 +75,27 @@ def test_init_dim_sizes_conflict(self) -> None:
         with pytest.raises(ValueError):
             Coordinates(coords={"foo": ("x", [1, 2]), "bar": ("x", [1, 2, 3, 4])})
 
+    def test_from_xindex(self) -> None:
+        idx = PandasIndex([1, 2, 3], "x")
+        coords = Coordinates.from_xindex(idx)
+
+        assert isinstance(coords.xindexes["x"], PandasIndex)
+        assert coords.xindexes["x"].equals(idx)
+
+        expected = PandasIndex(idx, "x").create_variables()
+        assert list(coords.variables) == list(expected)
+        assert_identical(expected["x"], coords.variables["x"])
+
+    def test_from_xindex_error(self) -> None:
+        class CustomIndexNoCoordsGenerated(Index):
+            def create_variables(self, variables: Mapping | None = None):
+                return {}
+
+        idx = CustomIndexNoCoordsGenerated()
+
+        with pytest.raises(ValueError, match=".*index.*did not create any coordinate"):
+            Coordinates.from_xindex(idx)
+
     def test_from_pandas_multiindex(self) -> None:
         midx = pd.MultiIndex.from_product([["a", "b"], [1, 2]], names=("one", "two"))
         coords = Coordinates.from_pandas_multiindex(midx, "x")