Backport PR #55832 on branch 2.1.x (BUG: Index.getitem returning wrong result with negative step for arrow) (#56121)

Author: phofl

doc/source/whatsnew/v2.1.4.rst

@@ -22,6 +22,7 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 - Bug in :meth:`DataFrame.apply` where passing ``raw=True`` ignored ``args`` passed to the applied function (:issue:`55753`)
+- Bug in :meth:`Index.__getitem__` returning wrong result for Arrow dtypes and negative stepsize (:issue:`55832`)
 - Fixed bug in :meth:`DataFrame.__setitem__` casting :class:`Index` with object-dtype to PyArrow backed strings when ``infer_string`` option is set (:issue:`55638`)
 - Fixed bug in :meth:`Index.insert` casting object-dtype to PyArrow backed strings when ``infer_string`` option is set (:issue:`55638`)
 -

pandas/core/arrays/arrow/array.py

@@ -552,6 +552,18 @@ def __getitem__(self, item: PositionalIndexer):
             )
         # We are not an array indexer, so maybe e.g. a slice or integer
         # indexer. We dispatch to pyarrow.
+        if isinstance(item, slice):
+            # Arrow bug https://github.com/apache/arrow/issues/38768
+            if item.start == item.stop:
+                pass
+            elif (
+                item.stop is not None
+                and item.stop < -len(self)
+                and item.step is not None
+                and item.step < 0
+            ):
+                item = slice(item.start, None, item.step)
+
         value = self._pa_array[item]
         if isinstance(value, pa.ChunkedArray):
             return type(self)(value)

pandas/tests/indexes/object/test_indexing.py

@@ -4,6 +4,7 @@
 import pytest
 
 from pandas._libs.missing import is_matching_na
+import pandas.util._test_decorators as td
 
 import pandas as pd
 from pandas import Index
@@ -144,6 +145,13 @@ def test_get_indexer_non_unique_np_nats(self, np_nat_fixture, np_nat_fixture2):
 
 
 class TestSliceLocs:
+    @pytest.mark.parametrize(
+        "dtype",
+        [
+            "object",
+            pytest.param("string[pyarrow_numpy]", marks=td.skip_if_no("pyarrow")),
+        ],
+    )
     @pytest.mark.parametrize(
         "in_slice,expected",
         [
@@ -167,12 +175,23 @@ class TestSliceLocs:
             (pd.IndexSlice["m":"m":-1], ""),  # type: ignore[misc]
         ],
     )
-    def test_slice_locs_negative_step(self, in_slice, expected):
-        index = Index(list("bcdxy"))
+    def test_slice_locs_negative_step(self, in_slice, expected, dtype):
+        index = Index(list("bcdxy"), dtype=dtype)
 
         s_start, s_stop = index.slice_locs(in_slice.start, in_slice.stop, in_slice.step)
         result = index[s_start : s_stop : in_slice.step]
-        expected = Index(list(expected))
+        expected = Index(list(expected), dtype=dtype)
+        tm.assert_index_equal(result, expected)
+
+    @td.skip_if_no("pyarrow")
+    def test_slice_locs_negative_step_oob(self):
+        index = Index(list("bcdxy"), dtype="string[pyarrow_numpy]")
+
+        result = index[-10:5:1]
+        tm.assert_index_equal(result, index)
+
+        result = index[4:-10:-1]
+        expected = Index(list("yxdcb"), dtype="string[pyarrow_numpy]")
         tm.assert_index_equal(result, expected)
 
     def test_slice_locs_dup(self):