Better default behavior of the Coordinates constructor (#8107)

* ``Coordinates.__init__`` create default indexes

... for any input dimension coordinate, if ``indexes=None``.

Also, if another ``Coordinates`` object is passed, extract its indexes
and raise if ``indexes`` is not None (no align/merge supported here).

* add docstring examples

* fix doctests

* fix tests

* update what's new

doc/whats-new.rst

@@ -26,6 +26,12 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- The :py:class:`Coordinates` constructor now creates a (pandas) index by
+  default for each dimension coordinate. To keep the previous behavior (no index
+  created), pass an empty dictionary to ``indexes``. The constructor now also
+  extracts and add the indexes from another :py:class:`Coordinates` object
+  passed via ``coords`` (:pull:`8107`).
+  By `Benoît Bovy <https://github.com/benbovy>`_.
 
 Deprecations
 ~~~~~~~~~~~~

xarray/core/coordinates.py

@@ -17,6 +17,7 @@
 from xarray.core.indexes import (
     Index,
     Indexes,
+    PandasIndex,
     PandasMultiIndex,
     assert_no_index_corrupted,
     create_default_index_implicit,
@@ -192,22 +193,69 @@ class Coordinates(AbstractCoordinates):
     Coordinates are either:
 
     - returned via the :py:attr:`Dataset.coords` and :py:attr:`DataArray.coords`
-      properties.
-    - built from index objects (e.g., :py:meth:`Coordinates.from_pandas_multiindex`).
-    - built directly from coordinate data and index objects (beware that no consistency
-      check is done on those inputs).
-
-    In the latter case, no default (pandas) index is created.
+      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)
 
     Parameters
     ----------
-    coords: dict-like
-         Mapping where keys are coordinate names and values are objects that
-         can be converted into a :py:class:`~xarray.Variable` object
-         (see :py:func:`~xarray.as_variable`).
-    indexes: dict-like
-         Mapping of where keys are coordinate names and values are
-         :py:class:`~xarray.indexes.Index` objects.
+    coords: dict-like, optional
+        Mapping where keys are coordinate names and values are objects that
+        can be converted into a :py:class:`~xarray.Variable` object
+        (see :py:func:`~xarray.as_variable`). If another
+        :py:class:`~xarray.Coordinates` object is passed, its indexes
+        will be added to the new created object.
+    indexes: dict-like, optional
+        Mapping of where keys are coordinate names and values are
+        :py:class:`~xarray.indexes.Index` objects. If None (default),
+        pandas indexes will be created for each dimension coordinate.
+        Passing an empty dictionary will skip this default behavior.
+
+    Examples
+    --------
+    Create a dimension coordinate with a default (pandas) index:
+
+    >>> xr.Coordinates({"x": [1, 2]})
+    Coordinates:
+      * x        (x) int64 1 2
+
+    Create a dimension coordinate with no index:
+
+    >>> xr.Coordinates(coords={"x": [1, 2]}, indexes={})
+    Coordinates:
+        x        (x) int64 1 2
+
+    Create a new Coordinates object from existing dataset coordinates
+    (indexes are passed):
+
+    >>> ds = xr.Dataset(coords={"x": [1, 2]})
+    >>> xr.Coordinates(ds.coords)
+    Coordinates:
+      * x        (x) int64 1 2
+
+    Create indexed coordinates from a ``pandas.MultiIndex`` object:
+
+    >>> midx = pd.MultiIndex.from_product([["a", "b"], [0, 1]])
+    >>> xr.Coordinates.from_pandas_multiindex(midx, "x")
+    Coordinates:
+      * x          (x) object MultiIndex
+      * x_level_0  (x) object 'a' 'a' 'b' 'b'
+      * x_level_1  (x) int64 0 1 0 1
+
+    Create a new Dataset object by passing a Coordinates object:
+
+    >>> midx_coords = xr.Coordinates.from_pandas_multiindex(midx, "x")
+    >>> xr.Dataset(coords=midx_coords)
+    <xarray.Dataset>
+    Dimensions:    (x: 4)
+    Coordinates:
+      * x          (x) object MultiIndex
+      * x_level_0  (x) object 'a' 'a' 'b' 'b'
+      * x_level_1  (x) int64 0 1 0 1
+    Data variables:
+        *empty*
 
     """
 
@@ -227,17 +275,40 @@ def __init__(
         from xarray.core.dataset import Dataset
 
         if coords is None:
-            variables = {}
-        elif isinstance(coords, Coordinates):
+            coords = {}
+
+        variables: dict[Hashable, Variable]
+        default_indexes: dict[Hashable, PandasIndex] = {}
+        coords_obj_indexes: dict[Hashable, Index] = {}
+
+        if isinstance(coords, Coordinates):
+            if indexes is not None:
+                raise ValueError(
+                    "passing both a ``Coordinates`` object and a mapping of indexes "
+                    "to ``Coordinates.__init__`` is not allowed "
+                    "(this constructor does not support merging them)"
+                )
             variables = {k: v.copy() for k, v in coords.variables.items()}
+            coords_obj_indexes = dict(coords.xindexes)
         else:
-            variables = {k: as_variable(v) for k, v in coords.items()}
+            variables = {}
+            for name, data in coords.items():
+                var = as_variable(data, name=name)
+                if var.dims == (name,) and indexes is None:
+                    index, index_vars = create_default_index_implicit(var, list(coords))
+                    default_indexes.update({k: index for k in index_vars})
+                    variables.update(index_vars)
+                else:
+                    variables[name] = var
 
         if indexes is None:
             indexes = {}
         else:
             indexes = dict(indexes)
 
+        indexes.update(default_indexes)
+        indexes.update(coords_obj_indexes)
+
         no_coord_index = set(indexes) - set(variables)
         if no_coord_index:
             raise ValueError(

xarray/tests/test_coordinates.py

@@ -17,6 +17,17 @@ def test_init_noindex(self) -> None:
         expected = Dataset(coords={"foo": ("x", [0, 1, 2])})
         assert_identical(coords.to_dataset(), expected)
 
+    def test_init_default_index(self) -> None:
+        coords = Coordinates(coords={"x": [1, 2]})
+        expected = Dataset(coords={"x": [1, 2]})
+        assert_identical(coords.to_dataset(), expected)
+        assert "x" in coords.xindexes
+
+    def test_init_no_default_index(self) -> None:
+        # dimension coordinate with no default index (explicit)
+        coords = Coordinates(coords={"x": [1, 2]}, indexes={})
+        assert "x" not in coords.xindexes
+
     def test_init_from_coords(self) -> None:
         expected = Dataset(coords={"foo": ("x", [0, 1, 2])})
         coords = Coordinates(coords=expected.coords)
@@ -25,10 +36,19 @@ def test_init_from_coords(self) -> None:
         # test variables copied
         assert coords.variables["foo"] is not expected.variables["foo"]
 
-        # default index
-        expected = Dataset(coords={"x": ("x", [0, 1, 2])})
-        coords = Coordinates(coords=expected.coords, indexes=expected.xindexes)
+        # test indexes are extracted
+        expected = Dataset(coords={"x": [0, 1, 2]})
+        coords = Coordinates(coords=expected.coords)
         assert_identical(coords.to_dataset(), expected)
+        assert expected.xindexes == coords.xindexes
+
+        # coords + indexes not supported
+        with pytest.raises(
+            ValueError, match="passing both.*Coordinates.*indexes.*not allowed"
+        ):
+            coords = Coordinates(
+                coords=expected.coords, indexes={"x": PandasIndex([0, 1, 2], "x")}
+            )
 
     def test_init_empty(self) -> None:
         coords = Coordinates()
@@ -60,37 +80,31 @@ def test_from_pandas_multiindex(self) -> None:
             assert_identical(expected[name], coords.variables[name])
 
     def test_dims(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
         assert coords.dims == {"x": 3}
 
     def test_sizes(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
         assert coords.sizes == {"x": 3}
 
     def test_dtypes(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
         assert coords.dtypes == {"x": int}
 
     def test_getitem(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
         assert_identical(
             coords["x"],
             DataArray([0, 1, 2], coords={"x": [0, 1, 2]}, name="x"),
         )
 
     def test_delitem(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
         del coords["x"]
         assert "x" not in coords
 
     def test_update(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
 
         coords.update({"y": ("y", [4, 5, 6])})
         assert "y" in coords
@@ -99,18 +113,16 @@ def test_update(self) -> None:
         assert_identical(coords["y"], expected)
 
     def test_equals(self):
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
 
         assert coords.equals(coords)
-        assert not coords.equals("no_a_coords")
+        assert not coords.equals("not_a_coords")
 
     def test_identical(self):
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
 
         assert coords.identical(coords)
-        assert not coords.identical("no_a_coords")
+        assert not coords.identical("not_a_coords")
 
     def test_copy(self) -> None:
         no_index_coords = Coordinates({"foo": ("x", [1, 2, 3])})
@@ -129,8 +141,7 @@ def test_copy(self) -> None:
         assert source_ndarray(v0.data) is not source_ndarray(v1.data)
 
     def test_align(self) -> None:
-        _ds = Dataset(coords={"x": [0, 1, 2]})
-        coords = Coordinates(coords=_ds.coords, indexes=_ds.xindexes)
+        coords = Coordinates(coords={"x": [0, 1, 2]})
 
         left = coords
 

xarray/tests/test_dataarray.py

@@ -489,7 +489,7 @@ def test_constructor_dask_coords(self) -> None:
 
     def test_constructor_no_default_index(self) -> None:
         # explicitly passing a Coordinates object skips the creation of default index
-        da = DataArray(range(3), coords=Coordinates({"x": ("x", [1, 2, 3])}))
+        da = DataArray(range(3), coords=Coordinates({"x": [1, 2, 3]}, indexes={}))
         assert "x" in da.coords
         assert "x" not in da.xindexes
 
@@ -1587,7 +1587,7 @@ class CustomIndex(Index):
         assert isinstance(actual.xindexes["x"], CustomIndex)
 
     def test_assign_coords_no_default_index(self) -> None:
-        coords = Coordinates({"y": ("y", [1, 2, 3])})
+        coords = Coordinates({"y": [1, 2, 3]}, indexes={})
         da = DataArray([1, 2, 3], dims="y")
         actual = da.assign_coords(coords)
         assert_identical(actual.coords, coords, check_default_indexes=False)

xarray/tests/test_dataset.py

@@ -636,7 +636,7 @@ def test_constructor_with_coords(self) -> None:
 
     def test_constructor_no_default_index(self) -> None:
         # explicitly passing a Coordinates object skips the creation of default index
-        ds = Dataset(coords=Coordinates({"x": ("x", [1, 2, 3])}))
+        ds = Dataset(coords=Coordinates({"x": [1, 2, 3]}, indexes={}))
         assert "x" in ds
         assert "x" not in ds.xindexes
 
@@ -4356,7 +4356,7 @@ class CustomIndex(Index):
         assert isinstance(actual.xindexes["x"], CustomIndex)
 
     def test_assign_coords_no_default_index(self) -> None:
-        coords = Coordinates({"y": ("y", [1, 2, 3])})
+        coords = Coordinates({"y": [1, 2, 3]}, indexes={})
         ds = Dataset()
         actual = ds.assign_coords(coords)
         expected = coords.to_dataset()