BUG: DataFrame.apply with result_type=reduce incorrect index

doc/source/whatsnew/v1.1.1.rst

@@ -45,6 +45,7 @@ Bug fixes
 - Bug in :meth:`to_timedelta` fails when arg is a :class:`Series` with `Int64` dtype containing null values (:issue:`35574`)
 - Bug in ``.groupby(..).rolling(..)`` where passing ``closed`` with column selection would raise a ``ValueError`` (:issue:`35549`)
 - Bug in :class:`DataFrame` constructor failing to raise ``ValueError`` in some cases when data and index have mismatched lengths (:issue:`33437`)
+- Bug in :meth:`DataFrame.apply` with ``result_type="reduce"`` returning with incorrect index (:issue:`35683`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/apply.py

@@ -340,7 +340,10 @@ def wrap_results_for_axis(
 
         if self.result_type == "reduce":
             # e.g. test_apply_dict GH#8735
-            return self.obj._constructor_sliced(results)
+            res = self.obj._constructor_sliced(results)
+            res.index = res_index
+            return res
+
         elif self.result_type is None and all(
             isinstance(x, dict) for x in results.values()
         ):

pandas/tests/frame/apply/test_frame_apply.py

@@ -1541,3 +1541,12 @@ def func(row):
 
     tm.assert_frame_equal(result, expected)
     tm.assert_frame_equal(df, result)
+
+
+def test_apply_empty_list_reduce():
+    # GH#35683 get columns correct
+    df = pd.DataFrame([[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]], columns=["a", "b"])
+
+    result = df.apply(lambda x: [], result_type="reduce")
+    expected = pd.Series({"a": [], "b": []}, dtype=object)
+    tm.assert_series_equal(result, expected)