Restrict fastpath isel indexes to the case of all PandasIndex (#10066)

Author: benbovy

Diff for: doc/whats-new.rst

@@ -55,6 +55,8 @@ Bug fixes
 - Fix DataArray().drop_attrs(deep=False) and add support for attrs to
   DataArray()._replace(). (:issue:`10027`, :pull:`10030`). By `Jan
   Haacker <https://github.com/j-haacker>`_.
+- Fix ``isel`` for multi-coordinate Xarray indexes (:issue:`10063`, :pull:`10066`).
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 
 
 Documentation

Diff for: xarray/core/indexes.py

@@ -1991,10 +1991,11 @@ def isel_indexes(
     indexes: Indexes[Index],
     indexers: Mapping[Any, Any],
 ) -> tuple[dict[Hashable, Index], dict[Hashable, Variable]]:
-    # TODO: remove if clause in the future. It should be unnecessary.
-    # See failure introduced when removed
-    # https://github.com/pydata/xarray/pull/9002#discussion_r1590443756
-    if any(isinstance(v, PandasMultiIndex) for v in indexes._indexes.values()):
+    # Fast path function _apply_indexes_fast does not work with multi-coordinate
+    # Xarray indexes (see https://github.com/pydata/xarray/issues/10063).
+    # -> call it only in the most common case where all indexes are default
+    # PandasIndex each associated to a single 1-dimensional coordinate.
+    if any(type(idx) is not PandasIndex for idx in indexes._indexes.values()):
         return _apply_indexes(indexes, indexers, "isel")
     else:
         return _apply_indexes_fast(indexes, indexers, "isel")

Diff for: xarray/tests/test_dataset.py

@@ -1593,6 +1593,40 @@ def test_isel_fancy_convert_index_variable(self) -> None:
         assert "x" not in actual.xindexes
         assert not isinstance(actual.x.variable, IndexVariable)
 
+    def test_isel_multicoord_index(self) -> None:
+        # regression test https://github.com/pydata/xarray/issues/10063
+        # isel on a multi-coordinate index should return a unique index associated
+        # to each coordinate
+        class MultiCoordIndex(xr.Index):
+            def __init__(self, idx1, idx2):
+                self.idx1 = idx1
+                self.idx2 = idx2
+
+            @classmethod
+            def from_variables(cls, variables, *, options=None):
+                idx1 = PandasIndex.from_variables(
+                    {"x": variables["x"]}, options=options
+                )
+                idx2 = PandasIndex.from_variables(
+                    {"y": variables["y"]}, options=options
+                )
+
+                return cls(idx1, idx2)
+
+            def create_variables(self, variables=None):
+                return {**self.idx1.create_variables(), **self.idx2.create_variables()}
+
+            def isel(self, indexers):
+                idx1 = self.idx1.isel({"x": indexers.get("x", slice(None))})
+                idx2 = self.idx2.isel({"y": indexers.get("y", slice(None))})
+                return MultiCoordIndex(idx1, idx2)
+
+        coords = xr.Coordinates(coords={"x": [0, 1], "y": [1, 2]}, indexes={})
+        ds = xr.Dataset(coords=coords).set_xindex(["x", "y"], MultiCoordIndex)
+
+        ds2 = ds.isel(x=slice(None), y=slice(None))
+        assert ds2.xindexes["x"] is ds2.xindexes["y"]
+
     def test_sel(self) -> None:
         data = create_test_data()
         int_slicers = {"dim1": slice(None, None, 2), "dim2": slice(2), "dim3": slice(3)}