avoid converting custom indexes to pandas indexes when formatting coordinate diffs (#9157)

* use `.xindexes` to construct the diff

* check that the indexes are never converted to pandas indexes

* deduplicate the various custom index dummy implementations

* whats-new [skip-rtd]

* fill in the pr number

---------

Co-authored-by: Deepak Cherian <[email protected]>

Author: keewis

doc/whats-new.rst

@@ -37,6 +37,8 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+- Don't convert custom indexes to ``pandas`` indexes when computing a diff (:pull:`9157`)
+  By `Justus Magin <https://github.com/keewis>`_.
 - Make :py:func:`testing.assert_allclose` work with numpy 2.0 (:issue:`9165`, :pull:`9166`).
   By `Pontus Lurcock <https://github.com/pont-us>`_.
 - Allow diffing objects with array attributes on variables (:issue:`9153`, :pull:`9169`).

xarray/core/formatting.py

@@ -898,8 +898,8 @@ def diff_coords_repr(a, b, compat, col_width=None):
         "Coordinates",
         summarize_variable,
         col_width=col_width,
-        a_indexes=a.indexes,
-        b_indexes=b.indexes,
+        a_indexes=a.xindexes,
+        b_indexes=b.xindexes,
     )
 
 

xarray/tests/test_formatting.py

@@ -10,9 +10,20 @@
 import xarray as xr
 from xarray.core import formatting
 from xarray.core.datatree import DataTree  # TODO: Remove when can do xr.DataTree
+from xarray.core.indexes import Index
 from xarray.tests import requires_cftime, requires_dask, requires_netCDF4
 
 
+class CustomIndex(Index):
+    names: tuple[str, ...]
+
+    def __init__(self, names: tuple[str, ...]):
+        self.names = names
+
+    def __repr__(self):
+        return f"CustomIndex(coords={self.names})"
+
+
 class TestFormatting:
     def test_get_indexer_at_least_n_items(self) -> None:
         cases = [
@@ -219,17 +230,6 @@ def test_attribute_repr(self) -> None:
         assert "\t" not in tabs
 
     def test_index_repr(self) -> None:
-        from xarray.core.indexes import Index
-
-        class CustomIndex(Index):
-            names: tuple[str, ...]
-
-            def __init__(self, names: tuple[str, ...]):
-                self.names = names
-
-            def __repr__(self):
-                return f"CustomIndex(coords={self.names})"
-
         coord_names = ("x", "y")
         index = CustomIndex(coord_names)
         names = ("x",)
@@ -258,15 +258,6 @@ def _repr_inline_(self, max_width: int):
         ),
     )
     def test_index_repr_grouping(self, names) -> None:
-        from xarray.core.indexes import Index
-
-        class CustomIndex(Index):
-            def __init__(self, names):
-                self.names = names
-
-            def __repr__(self):
-                return f"CustomIndex(coords={self.names})"
-
         index = CustomIndex(names)
 
         normal = formatting.summarize_index(names, index, col_width=20)
@@ -337,6 +328,51 @@ def test_diff_array_repr(self) -> None:
             # depending on platform, dtype may not be shown in numpy array repr
             assert actual == expected.replace(", dtype=int64", "")
 
+        da_a = xr.DataArray(
+            np.array([[1, 2, 3], [4, 5, 6]], dtype="int8"),
+            dims=("x", "y"),
+            coords=xr.Coordinates(
+                {
+                    "x": np.array([True, False], dtype="bool"),
+                    "y": np.array([1, 2, 3], dtype="int16"),
+                },
+                indexes={"y": CustomIndex(("y",))},
+            ),
+        )
+
+        da_b = xr.DataArray(
+            np.array([1, 2], dtype="int8"),
+            dims="x",
+            coords=xr.Coordinates(
+                {
+                    "x": np.array([True, False], dtype="bool"),
+                    "label": ("x", np.array([1, 2], dtype="int16")),
+                },
+                indexes={"label": CustomIndex(("label",))},
+            ),
+        )
+
+        expected = dedent(
+            """\
+            Left and right DataArray objects are not equal
+            Differing dimensions:
+                (x: 2, y: 3) != (x: 2)
+            Differing values:
+            L
+                array([[1, 2, 3],
+                       [4, 5, 6]], dtype=int8)
+            R
+                array([1, 2], dtype=int8)
+            Coordinates only on the left object:
+              * y        (y) int16 6B 1 2 3
+            Coordinates only on the right object:
+              * label    (x) int16 4B 1 2
+            """.rstrip()
+        )
+
+        actual = formatting.diff_array_repr(da_a, da_b, "equals")
+        assert actual == expected
+
         va = xr.Variable(
             "x", np.array([1, 2, 3], dtype="int64"), {"title": "test Variable"}
         )