slicing impl: rely on range

This also enables support for reverse slicing (added tests)
pydata · dcherian · Jul 1, 2025 · Jun 30, 2025 · Jul 1, 2025 · Jul 1, 2025
commit ea6742cac4cb16f3b1a68f2f01076e5bda6a95a2
diff --git a/xarray/indexes/range_index.py b/xarray/indexes/range_index.py
@@ -9,7 +9,7 @@
 from xarray.core.coordinate_transform import CoordinateTransform
 from xarray.core.dataarray import DataArray
 from xarray.core.indexes import CoordinateTransformIndex, Index, PandasIndex
-from xarray.core.indexing import IndexSelResult, normalize_slice
+from xarray.core.indexing import IndexSelResult
 from xarray.core.variable import Variable
 
 
@@ -79,14 +79,10 @@ def equals(
         )
 
     def slice(self, sl: slice) -> "RangeCoordinateTransform":
-        sl = normalize_slice(sl, self.size)
-
-        # TODO: support reverse transform (i.e., start > stop)?
-        assert sl.start < sl.stop
-
-        new_size = (sl.stop - sl.start - 1) // sl.step + 1
-        new_start = self.start + sl.start * self.step
-        new_stop = min(new_start + new_size * sl.step * self.step, self.stop)
+        new_range = range(self.size)[sl]
+        new_size = len(new_range)
+        new_start = self.start + new_range.start * self.step
+        new_stop = self.start + new_range.stop * self.step
 
         return type(self)(
             new_start, new_stop, new_size, self.coord_name, self.dim, dtype=self.dtype

diff --git a/xarray/tests/test_range_index.py b/xarray/tests/test_range_index.py
@@ -121,6 +121,22 @@ def test_range_index_isel() -> None:
     expected = create_dataset_arange(0.0, 1.0, 0.2)
     assert_identical(actual, expected, check_default_indexes=False)
 
+    actual = ds.isel(x=slice(None, None, -1))
+    expected = create_dataset_arange(0.9, -0.1, -0.1)
+    assert_identical(actual, expected, check_default_indexes=False)
+
+    actual = ds.isel(x=slice(None, 4, -1))
+    expected = create_dataset_arange(0.9, 0.4, -0.1)
+    assert_identical(actual, expected, check_default_indexes=False)
+
+    actual = ds.isel(x=slice(8, 4, -1))
+    expected = create_dataset_arange(0.8, 0.4, -0.1)
+    assert_identical(actual, expected, check_default_indexes=False)
+
+    actual = ds.isel(x=slice(8, None, -1))
+    expected = create_dataset_arange(0.8, -0.1, -0.1)
+    assert_identical(actual, expected, check_default_indexes=False)
+
     # https://github.com/pydata/xarray/issues/10441
     ds2 = create_dataset_arange(0.0, 3.0, 0.1)
     actual = ds2.isel(x=slice(4, None, 3))