More permissive Index typing (#10067)

* Index.isel: more permissive return type

This allows an index to return a new index of another type, e.g., a
1-dimensional CoordinateTransformIndex to return a PandasIndex when a
new transform cannot be computed (selection at arbitrary locations).

* Index.equals: more permissive `other` type.

Xarray alignment logic is such that Xarray indexes are always compared
with other indexes of the same type. However, this is not necessarily
the case for "meta" indexes (i.e., indexes encapsulating one or more
index objects that may have another type) that are dispatching `equals`
to their wrapped indexes.

Author: benbovy

xarray/core/indexes.py

@@ -209,7 +209,7 @@ def to_pandas_index(self) -> pd.Index:
 
     def isel(
         self, indexers: Mapping[Any, int | slice | np.ndarray | Variable]
-    ) -> Self | None:
+    ) -> Index | None:
         """Maybe returns a new index from the current index itself indexed by
         positional indexers.
 
@@ -304,7 +304,7 @@ def reindex_like(self, other: Self) -> dict[Hashable, Any]:
         """
         raise NotImplementedError(f"{self!r} doesn't support re-indexing labels")
 
-    def equals(self, other: Self) -> bool:
+    def equals(self, other: Index) -> bool:
         """Compare this index with another index of the same type.
 
         Implementation is optional but required in order to support alignment.
@@ -1424,7 +1424,7 @@ def create_variables(
 
     def isel(
         self, indexers: Mapping[Any, int | slice | np.ndarray | Variable]
-    ) -> Self | None:
+    ) -> Index | None:
         # TODO: support returning a new index (e.g., possible to re-calculate the
         # the transform or calculate another transform on a reduced dimension space)
         return None
@@ -1486,7 +1486,9 @@ def sel(
 
         return IndexSelResult(results)
 
-    def equals(self, other: Self) -> bool:
+    def equals(self, other: Index) -> bool:
+        if not isinstance(other, CoordinateTransformIndex):
+            return False
         return self.transform.equals(other.transform)
 
     def rename(