BUG: Series.setitem raising ValueError when setting Series with scalar indexer

doc/source/whatsnew/v1.3.0.rst

@@ -287,6 +287,7 @@ Indexing
 - Bug in :meth:`DataFrame.__setitem__` raising ``ValueError`` with empty :class:`DataFrame` and specified columns for string indexer and non empty :class:`DataFrame` to set (:issue:`38831`)
 - Bug in :meth:`DataFrame.loc.__setitem__` raising ValueError when expanding unique column for :class:`DataFrame` with duplicate columns (:issue:`38521`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` and :meth:`DataFrame.loc.__setitem__` with mixed dtypes when setting with a dictionary value (:issue:`38335`)
+- Bug in :meth:`Series.__setitem__` raising ``ValueError`` when setting a :class:`Series` with a scalar indexer (:issue:`38303`)
 - Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
 - Bug in setting ``timedelta64`` values into numeric :class:`Series` failing to cast to object dtype (:issue:`39086`)
 - Bug in setting :class:`Interval` values into a :class:`Series` or :class:`DataFrame` with mismatched :class:`IntervalDtype` incorrectly casting the new values to the existing dtype (:issue:`39120`)

pandas/core/indexing.py

@@ -2023,6 +2023,16 @@ def ravel(i):
                         return ser._values.copy()
                     return ser.reindex(ax)._values
 
+        elif is_scalar(indexer) and isinstance(self.obj, ABCSeries):
+            if is_object_dtype(self.obj):
+                return ser
+            ax = self.obj._get_axis(0)
+
+            if ser.index.equals(ax):
+                return ser._values.copy()
+
+            return ser.reindex(ax)._values[indexer]
+
         elif is_scalar(indexer):
             ax = self.obj._get_axis(1)
 

pandas/tests/series/indexing/test_setitem.py

@@ -93,6 +93,22 @@ def test_setitem_negative_out_of_bounds(self):
         with pytest.raises(IndexError, match=msg):
             ser[-11] = "foo"
 
+    @pytest.mark.parametrize("ser_index", [0, 1])
+    def test_setitem_series_object_dtype(self, ser_index):
+        # GH#38303
+        ser = Series([0, 0], dtype="object")
+        ser.loc[0] = Series([42], index=[ser_index])
+        expected = Series([Series([42], index=[ser_index]), 0], dtype="object")
+        tm.assert_series_equal(ser, expected)
+
+    @pytest.mark.parametrize("index, exp_value", [(0, 42.0), (1, np.nan)])
+    def test_setitem_series(self, index, exp_value):
+        # GH#38303
+        ser = Series([0, 0])
+        ser.loc[0] = Series([42], index=[index])
+        expected = Series([exp_value, 0])
+        tm.assert_series_equal(ser, expected)
+
 
 class TestSetitemSlices:
     def test_setitem_slice_float_raises(self, datetime_series):