API (string): str.center with pyarrow-backed string dtype (#59624)

Author: jbrockmendel

doc/source/whatsnew/v2.3.0.rst

@@ -103,7 +103,8 @@ Conversion
 Strings
 ^^^^^^^
 - Bug in :meth:`Series.str.replace` when ``n < 0`` for :class:`StringDtype` with ``storage="pyarrow"`` (:issue:`59628`)
--
+- 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

@@ -1,13 +1,17 @@
 from __future__ import annotations
 
+from functools import partial
 from typing import (
     TYPE_CHECKING,
     Literal,
 )
 
 import numpy as np
 
-from pandas.compat import pa_version_under10p1
+from pandas.compat import (
+    pa_version_under10p1,
+    pa_version_under17p0,
+)
 
 from pandas.core.dtypes.missing import isna
 
@@ -46,7 +50,19 @@ def _str_pad(
         elif side == "right":
             pa_pad = pc.utf8_rpad
         elif side == "both":
-            pa_pad = pc.utf8_center
+            if pa_version_under17p0:
+                # GH#59624 fall back to object dtype
+                from pandas import array
+
+                obj_arr = self.astype(object, copy=False)  # type: ignore[attr-defined]
+                obj = array(obj_arr, dtype=object)
+                result = obj._str_pad(width, side, fillchar)  # type: ignore[attr-defined]
+                return type(self)._from_sequence(result, dtype=self.dtype)  # type: ignore[attr-defined]
+            else:
+                # GH#54792
+                # https://github.com/apache/arrow/issues/15053#issuecomment-2317032347
+                lean_left = (width % 2) == 0
+                pa_pad = partial(pc.utf8_center, lean_left_on_odd_padding=lean_left)
         else:
             raise ValueError(
                 f"Invalid side: {side}. Side must be one of 'left', 'right', 'both'"

pandas/core/arrays/string_arrow.py

@@ -286,6 +286,7 @@ def _data(self):
     _str_map = BaseStringArray._str_map
     _str_startswith = ArrowStringArrayMixin._str_startswith
     _str_endswith = ArrowStringArrayMixin._str_endswith
+    _str_pad = ArrowStringArrayMixin._str_pad
 
     def _str_contains(
         self, pat, case: bool = True, flags: int = 0, na=np.nan, regex: bool = True
@@ -546,7 +547,6 @@ class ArrowStringArrayNumpySemantics(ArrowStringArray):
     _str_get = ArrowStringArrayMixin._str_get
     _str_removesuffix = ArrowStringArrayMixin._str_removesuffix
     _str_capitalize = ArrowStringArrayMixin._str_capitalize
-    _str_pad = ArrowStringArrayMixin._str_pad
     _str_title = ArrowStringArrayMixin._str_title
     _str_swapcase = ArrowStringArrayMixin._str_swapcase
     _str_slice_replace = ArrowStringArrayMixin._str_slice_replace

pandas/tests/strings/test_case_justify.py

@@ -291,11 +291,7 @@ def test_center_ljust_rjust_mixed_object():
 
 
 def test_center_ljust_rjust_fillchar(any_string_dtype):
-    if any_string_dtype == "string[pyarrow_numpy]":
-        pytest.skip(
-            "Arrow logic is different, "
-            "see https://github.com/pandas-dev/pandas/pull/54533/files#r1299808126",
-        )
+    # GH#54533, GH#54792
     s = Series(["a", "bb", "cccc", "ddddd", "eeeeee"], dtype=any_string_dtype)
 
     result = s.str.center(5, fillchar="X")