From ab076c5a694127b57cd5c7c1d705004c72ee8a1b Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Thu, 30 Nov 2023 18:45:43 +0100
Subject: [PATCH] BUG: mode not preserving object dtype for string option
 (#56184)

* BUG: mode not preserving object dtype for string option

* Adjust tests in root directory for new string option

(cherry picked from commit 59a17582bf2e74df735fc7dc90e80c5b441fd299)
---
 doc/source/whatsnew/v2.1.4.rst         |  1 +
 pandas/core/series.py                  |  6 +++++-
 pandas/tests/series/test_reductions.py | 10 ++++++++++
 pandas/tests/test_algos.py             |  2 +-
 4 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v2.1.4.rst b/doc/source/whatsnew/v2.1.4.rst
index 6cd44d954a708..8168cb7910e5e 100644
--- a/doc/source/whatsnew/v2.1.4.rst
+++ b/doc/source/whatsnew/v2.1.4.rst
@@ -30,6 +30,7 @@ Bug fixes
 - 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:`DataFrame.to_hdf` raising when columns have ``StringDtype`` (:issue:`55088`)
 - Fixed bug in :meth:`Index.insert` casting object-dtype to PyArrow backed strings when ``infer_string`` option is set (:issue:`55638`)
+- Fixed bug in :meth:`Series.mode` not keeping object dtype when ``infer_string`` is set (:issue:`56183`)
 - Fixed bug in :meth:`Series.str.split` and :meth:`Series.str.rsplit` when ``pat=None`` for :class:`ArrowDtype` with ``pyarrow.string`` (:issue:`56271`)
 - Fixed bug in :meth:`Series.str.translate` losing object dtype when string option is set (:issue:`56152`)
 -
diff --git a/pandas/core/series.py b/pandas/core/series.py
index 7b22d89bfe22d..956515aeaf289 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -2218,7 +2218,11 @@ def mode(self, dropna: bool = True) -> Series:
 
         # Ensure index is type stable (should always use int index)
         return self._constructor(
-            res_values, index=range(len(res_values)), name=self.name, copy=False
+            res_values,
+            index=range(len(res_values)),
+            name=self.name,
+            copy=False,
+            dtype=self.dtype,
         ).__finalize__(self, method="mode")
 
     def unique(self) -> ArrayLike:  # pylint: disable=useless-parent-delegation
diff --git a/pandas/tests/series/test_reductions.py b/pandas/tests/series/test_reductions.py
index 1e1ac100b21bf..75c7359fa81c0 100644
--- a/pandas/tests/series/test_reductions.py
+++ b/pandas/tests/series/test_reductions.py
@@ -29,6 +29,16 @@ def test_mode_extension_dtype(as_period):
     tm.assert_series_equal(res, ser)
 
 
+def test_mode_infer_string():
+    # GH#56183
+    pytest.importorskip("pyarrow")
+    ser = Series(["a", "b"], dtype=object)
+    with pd.option_context("future.infer_string", True):
+        result = ser.mode()
+    expected = Series(["a", "b"], dtype=object)
+    tm.assert_series_equal(result, expected)
+
+
 def test_reductions_td64_with_nat():
     # GH#8617
     ser = Series([0, pd.NaT], dtype="m8[ns]")
diff --git a/pandas/tests/test_algos.py b/pandas/tests/test_algos.py
index cb703d3439d44..c9aa410fed946 100644
--- a/pandas/tests/test_algos.py
+++ b/pandas/tests/test_algos.py
@@ -2102,7 +2102,7 @@ def test_timedelta_mode(self):
         tm.assert_series_equal(ser.mode(), exp)
 
     def test_mixed_dtype(self):
-        exp = Series(["foo"])
+        exp = Series(["foo"], dtype=object)
         ser = Series([1, "foo", "foo"])
         tm.assert_numpy_array_equal(algos.mode(ser.values), exp.values)
         tm.assert_series_equal(ser.mode(), exp)