BUG (string): Series.str.slice with negative step (#59724)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: jbrockmendel

doc/source/whatsnew/v2.3.0.rst

@@ -103,8 +103,9 @@ Conversion
 Strings
 ^^^^^^^
 - Bug in :meth:`Series.str.replace` when ``n < 0`` for :class:`StringDtype` with ``storage="pyarrow"`` (:issue:`59628`)
+- Bug in ``ser.str.slice`` with negative ``step`` with :class:`ArrowDtype` and :class:`StringDtype` with ``storage="pyarrow"`` giving incorrect results (:issue:`59710`)
 - Bug in the ``center`` method on :class:`Series` and :class:`Index` object ``str`` accessors with pyarrow-backed dtype not matching the python behavior in corner cases with an odd number of fill characters (:issue:`54792`)
-
+-
 
 Interval
 ^^^^^^^^

pandas/core/arrays/_arrow_string_mixins.py

@@ -11,6 +11,7 @@
 
 from pandas.compat import (
     pa_version_under10p1,
+    pa_version_under11p0,
     pa_version_under13p0,
     pa_version_under17p0,
 )
@@ -22,10 +23,7 @@
     import pyarrow.compute as pc
 
 if TYPE_CHECKING:
-    from collections.abc import (
-        Callable,
-        Sized,
-    )
+    from collections.abc import Callable
 
     from pandas._typing import (
         Scalar,
@@ -34,7 +32,7 @@
 
 
 class ArrowStringArrayMixin:
-    _pa_array: Sized
+    _pa_array: pa.ChunkedArray
 
     def __init__(self, *args, **kwargs) -> None:
         raise NotImplementedError
@@ -96,13 +94,29 @@ def _str_get(self, i: int) -> Self:
         selected = pc.utf8_slice_codeunits(
             self._pa_array, start=start, stop=stop, step=step
         )
-        null_value = pa.scalar(
-            None,
-            type=self._pa_array.type,  # type: ignore[attr-defined]
-        )
+        null_value = pa.scalar(None, type=self._pa_array.type)
         result = pc.if_else(not_out_of_bounds, selected, null_value)
         return type(self)(result)
 
+    def _str_slice(
+        self, start: int | None = None, stop: int | None = None, step: int | None = None
+    ) -> Self:
+        if pa_version_under11p0:
+            # GH#59724
+            result = self._apply_elementwise(lambda val: val[start:stop:step])
+            return type(self)(pa.chunked_array(result, type=self._pa_array.type))
+        if start is None:
+            if step is not None and step < 0:
+                # GH#59710
+                start = -1
+            else:
+                start = 0
+        if step is None:
+            step = 1
+        return type(self)(
+            pc.utf8_slice_codeunits(self._pa_array, start=start, stop=stop, step=step)
+        )
+
     def _str_slice_replace(
         self, start: int | None = None, stop: int | None = None, repl: str | None = None
     ) -> Self:

pandas/core/arrays/arrow/array.py

@@ -2394,17 +2394,6 @@ def _str_rpartition(self, sep: str, expand: bool) -> Self:
         result = self._apply_elementwise(predicate)
         return type(self)(pa.chunked_array(result))
 
-    def _str_slice(
-        self, start: int | None = None, stop: int | None = None, step: int | None = None
-    ) -> Self:
-        if start is None:
-            start = 0
-        if step is None:
-            step = 1
-        return type(self)(
-            pc.utf8_slice_codeunits(self._pa_array, start=start, stop=stop, step=step)
-        )
-
     def _str_len(self) -> Self:
         return type(self)(pc.utf8_length(self._pa_array))
 

pandas/core/arrays/string_arrow.py

@@ -294,6 +294,7 @@ def astype(self, dtype, copy: bool = True):
     _str_startswith = ArrowStringArrayMixin._str_startswith
     _str_endswith = ArrowStringArrayMixin._str_endswith
     _str_pad = ArrowStringArrayMixin._str_pad
+    _str_slice = ArrowStringArrayMixin._str_slice
 
     def _str_contains(
         self, pat, case: bool = True, flags: int = 0, na=np.nan, regex: bool = True
@@ -352,19 +353,6 @@ def _str_fullmatch(
             pat = f"{pat}$"
         return self._str_match(pat, case, flags, na)
 
-    def _str_slice(
-        self, start: int | None = None, stop: int | None = None, step: int | None = None
-    ) -> Self:
-        if stop is None:
-            return super()._str_slice(start, stop, step)
-        if start is None:
-            start = 0
-        if step is None:
-            step = 1
-        return type(self)(
-            pc.utf8_slice_codeunits(self._pa_array, start=start, stop=stop, step=step)
-        )
-
     def _str_len(self):
         result = pc.utf8_length(self._pa_array)
         return self._convert_int_result(result)

pandas/tests/extension/test_arrow.py

@@ -2036,6 +2036,7 @@ def test_str_join_string_type():
         [None, 2, None, ["ab", None]],
         [None, 2, 1, ["ab", None]],
         [1, 3, 1, ["bc", None]],
+        (None, None, -1, ["dcba", None]),
     ],
 )
 def test_str_slice(start, stop, step, exp):

pandas/tests/strings/test_strings.py

@@ -394,6 +394,7 @@ def test_pipe_failures(any_string_dtype):
         (2, 5, None, ["foo", "bar", np.nan, "baz"]),
         (0, 3, -1, ["", "", np.nan, ""]),
         (None, None, -1, ["owtoofaa", "owtrabaa", np.nan, "xuqzabaa"]),
+        (None, 2, -1, ["owtoo", "owtra", np.nan, "xuqza"]),
         (3, 10, 2, ["oto", "ato", np.nan, "aqx"]),
         (3, 0, -1, ["ofa", "aba", np.nan, "aba"]),
     ],