Improve error message for missing coordinate index (#9370)

* fix GH9201

Provide a more informative error message, specifically which coordinate is without index.

* add simple test

* fix ValueError in combine_by_coords test

Author: nicrie

xarray/core/combine.py

@@ -89,10 +89,12 @@ def _infer_concat_order_from_coords(datasets):
             # Need to read coordinate values to do ordering
             indexes = [ds._indexes.get(dim) for ds in datasets]
             if any(index is None for index in indexes):
-                raise ValueError(
-                    "Every dimension needs a coordinate for "
-                    "inferring concatenation order"
+                error_msg = (
+                    f"Every dimension requires a corresponding 1D coordinate "
+                    f"and index for inferring concatenation order but the "
+                    f"coordinate '{dim}' has no corresponding index"
                 )
+                raise ValueError(error_msg)
 
             # TODO (benbovy, flexible indexes): support flexible indexes?
             indexes = [index.to_pandas_index() for index in indexes]

xarray/tests/test_combine.py

@@ -728,7 +728,10 @@ def test_combine_by_coords(self):
             combine_by_coords(objs)
 
         objs = [Dataset({"x": [0], "y": [0]}), Dataset({"x": [0]})]
-        with pytest.raises(ValueError, match=r"Every dimension needs a coordinate"):
+        with pytest.raises(
+            ValueError,
+            match=r"Every dimension requires a corresponding 1D coordinate and index",
+        ):
             combine_by_coords(objs)
 
     def test_empty_input(self):