BUG: set index of DataFrame.apply(f) when f returns dict (#37544) (#37606)

Author: ma3da

doc/source/whatsnew/v1.2.0.rst

@@ -547,6 +547,7 @@ Reshaping
 - Bug in :func:`join` returned a non deterministic level-order for the resulting :class:`MultiIndex` (:issue:`36910`)
 - Bug in :meth:`DataFrame.combine_first()` caused wrong alignment with dtype ``string`` and one level of ``MultiIndex`` containing only ``NA`` (:issue:`37591`)
 - Fixed regression in :func:`merge` on merging DatetimeIndex with empty DataFrame (:issue:`36895`)
+- Bug in :meth:`DataFrame.apply` not setting index of return value when ``func`` return type is ``dict`` (:issue:`37544`)
 
 Sparse
 ^^^^^^

pandas/core/apply.py

@@ -362,8 +362,10 @@ def wrap_results_for_axis(
             isinstance(x, dict) for x in results.values()
         ):
             # Our operation was a to_dict op e.g.
-            #  test_apply_dict GH#8735, test_apply_reduce_rows_to_dict GH#25196
-            return self.obj._constructor_sliced(results)
+            #  test_apply_dict GH#8735, test_apply_reduce_to_dict GH#25196 #37544
+            res = self.obj._constructor_sliced(results)
+            res.index = res_index
+            return res
 
         try:
             result = self.obj._constructor(data=results)

pandas/tests/frame/apply/test_frame_apply.py

@@ -356,12 +356,17 @@ def test_apply_reduce_Series(self, float_frame):
         result = float_frame.apply(np.mean, axis=1)
         tm.assert_series_equal(result, expected)
 
-    def test_apply_reduce_rows_to_dict(self):
-        # GH 25196
-        data = DataFrame([[1, 2], [3, 4]])
-        expected = Series([{0: 1, 1: 3}, {0: 2, 1: 4}])
-        result = data.apply(dict)
-        tm.assert_series_equal(result, expected)
+    def test_apply_reduce_to_dict(self):
+        # GH 25196 37544
+        data = DataFrame([[1, 2], [3, 4]], columns=["c0", "c1"], index=["i0", "i1"])
+
+        result0 = data.apply(dict, axis=0)
+        expected0 = Series([{"i0": 1, "i1": 3}, {"i0": 2, "i1": 4}], index=data.columns)
+        tm.assert_series_equal(result0, expected0)
+
+        result1 = data.apply(dict, axis=1)
+        expected1 = Series([{"c0": 1, "c1": 2}, {"c0": 3, "c1": 4}], index=data.index)
+        tm.assert_series_equal(result1, expected1)
 
     def test_apply_differently_indexed(self):
         df = DataFrame(np.random.randn(20, 10))