Backport PR #56160 on branch 2.1.x (BUG: reset_index not preserving object dtype for string option) (#56387)

Author: phofl

doc/source/whatsnew/v2.1.4.rst

@@ -32,6 +32,7 @@ Bug fixes
 - 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.reset_index` not preserving object dtype when ``infer_string`` is set (:issue:`56160`)
 - 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

@@ -1652,7 +1652,7 @@ def reset_index(
                 return new_ser.__finalize__(self, method="reset_index")
             else:
                 return self._constructor(
-                    self._values.copy(), index=new_index, copy=False
+                    self._values.copy(), index=new_index, copy=False, dtype=self.dtype
                 ).__finalize__(self, method="reset_index")
         elif inplace:
             raise TypeError(

pandas/tests/series/methods/test_reset_index.py

@@ -11,6 +11,7 @@
     RangeIndex,
     Series,
     date_range,
+    option_context,
 )
 import pandas._testing as tm
 
@@ -155,6 +156,14 @@ def test_reset_index_inplace_and_drop_ignore_name(self):
         expected = Series(range(2), name="old")
         tm.assert_series_equal(ser, expected)
 
+    def test_reset_index_drop_infer_string(self):
+        # GH#56160
+        pytest.importorskip("pyarrow")
+        ser = Series(["a", "b", "c"], dtype=object)
+        with option_context("future.infer_string", True):
+            result = ser.reset_index(drop=True)
+        tm.assert_series_equal(result, ser)
+
 
 @pytest.mark.parametrize(
     "array, dtype",