Better default behavior of the Coordinates constructor (pydata#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

Author: benbovy

xarray/core/coordinates.py

@@ -296,9 +296,15 @@ def __init__(
             variables = {k: v.copy() for k, v in coords.variables.items()}
             coords_obj_indexes = dict(coords.xindexes)
         else:
-            variables = {
-                k: as_variable(v, name=k, auto_convert=False) for k, v in coords.items()
-            }
+            variables = {}
+            for name, data in coords.items():
+                var = as_variable(data, name=name, auto_convert=False)
+                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 = {}

xarray/tests/test_coordinates.py

@@ -82,7 +82,7 @@ def test_from_pandas_multiindex(self) -> None:
     @pytest.mark.filterwarnings("ignore:return type")
     def test_dims(self) -> None:
         coords = Coordinates(coords={"x": [0, 1, 2]})
-        assert set(coords.dims) == {"x"}
+        assert coords.dims == {"x": 3}
 
     def test_sizes(self) -> None:
         coords = Coordinates(coords={"x": [0, 1, 2]})