Backport PR #56184 on branch 2.1.x (BUG: mode not preserving object dtype for string option) (#56383)

Author: phofl

doc/source/whatsnew/v2.1.4.rst

@@ -31,6 +31,7 @@ Bug fixes
 - 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.__ne__` resulting in False for comparison between ``NA`` and string value for ``dtype="string[pyarrow_numpy]"`` (:issue:`56122`)
+- 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`)
 -

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

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]")

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)