Backport PR #55534 on branch 2.1.x (BUG: Ensure "string[pyarrow]" type is preserved when calling extractall) (#55597)

Backport PR #55534: BUG: Ensure "string[pyarrow]" type is preserved when calling extractall

Co-authored-by: Amanda Bizzinotto <amandabizzinotto@gmail.com>

Author: meeseeksmachine

doc/source/whatsnew/v2.1.2.rst

@@ -31,6 +31,7 @@ Bug fixes
 - Fixed bug in :meth:`Series.all`  and :meth:`Series.any` not treating missing values correctly for ``dtype="string[pyarrow_numpy]"`` (:issue:`55367`)
 - Fixed bug in :meth:`Series.floordiv` for :class:`ArrowDtype` (:issue:`55561`)
 - Fixed bug in :meth:`Series.rank` for ``string[pyarrow_numpy]`` dtype (:issue:`55362`)
+- Fixed bug in :meth:`Series.str.extractall` for :class:`ArrowDtype` dtype being converted to object (:issue:`53846`)
 - Silence ``Period[B]`` warnings introduced by :issue:`53446` during normal plotting activity (:issue:`55138`)
 
 .. ---------------------------------------------------------------------------

pandas/core/strings/accessor.py

@@ -3449,10 +3449,9 @@ def _result_dtype(arr):
     # when the list of values is empty.
     from pandas.core.arrays.string_ import StringDtype
 
-    if isinstance(arr.dtype, StringDtype):
+    if isinstance(arr.dtype, (ArrowDtype, StringDtype)):
         return arr.dtype
-    else:
-        return object
+    return object
 
 
 def _get_single_group_name(regex: re.Pattern) -> Hashable:

pandas/tests/strings/test_extract.py

@@ -4,6 +4,8 @@
 import numpy as np
 import pytest
 
+from pandas.core.dtypes.dtypes import ArrowDtype
+
 from pandas import (
     DataFrame,
     Index,
@@ -706,3 +708,12 @@ def test_extractall_same_as_extract_subject_index(any_string_dtype):
     has_match_index = s.str.extractall(pattern_one_noname)
     no_match_index = has_match_index.xs(0, level="match")
     tm.assert_frame_equal(extract_one_noname, no_match_index)
+
+
+def test_extractall_preserves_dtype():
+    # Ensure that when extractall is called on a series with specific dtypes set, that
+    # the dtype is preserved in the resulting DataFrame's column.
+    pa = pytest.importorskip("pyarrow")
+
+    result = Series(["abc", "ab"], dtype=ArrowDtype(pa.string())).str.extractall("(ab)")
+    assert result.dtypes[0] == "string[pyarrow]"