fix based on review comments

Author: matiaslindgren

doc/source/whatsnew/v3.0.0.rst

@@ -28,6 +28,7 @@ enhancement2
 
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
+- Fix confusing chain of ``KeyError`` exceptions (:issue:`58807`) when :meth:`DataFrame.agg` is called with ``axis=1`` and a ``func`` which relabels the result index.
 - :class:`pandas.api.typing.FrozenList` is available for typing the outputs of :attr:`MultiIndex.names`, :attr:`MultiIndex.codes` and :attr:`MultiIndex.levels` (:issue:`58237`)
 - :class:`pandas.api.typing.SASReader` is available for typing the output of :func:`read_sas` (:issue:`55689`)
 - :func:`DataFrame.to_excel` now raises an ``UserWarning`` when the character count in a cell exceeds Excel's limitation of 32767 characters (:issue:`56954`)
@@ -116,18 +117,9 @@ These improvements also fixed certain bugs in groupby:
 - :meth:`.DataFrameGroupBy.sum` would have incorrect values when there are multiple groupings, unobserved groups, and non-numeric data (:issue:`43891`)
 - :meth:`.DataFrameGroupBy.value_counts` would produce incorrect results when used with some categorical and some non-categorical groupings and ``observed=False`` (:issue:`56016`)
 
-.. _whatsnew_300.notable_bug_fixes.improved_error_message_agg:
+.. _whatsnew_300.notable_bug_fixes.notable_bug_fix2:
 
-Improved error message for :meth:`DataFrame.agg`
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-When :meth:`DataFrame.agg` is called with ``axis=1`` and a ``func`` which relabels the result index, the :func:`relabel_result` function iterates over result columns rather than the result rows, causing a confusing chain of ``KeyError`` exceptions (:issue:`58807`).
-
-This error is now handled in :func:`frame_apply` by throwing a ``NotImplementedError`` with a more explicit error message.
-
-.. _whatsnew_300.notable_bug_fixes.notable_bug_fix3:
-
-notable_bug_fix3
+notable_bug_fix2
 ^^^^^^^^^^^^^^^^
 
 .. ---------------------------------------------------------------------------

pandas/core/apply.py

@@ -99,8 +99,7 @@ def frame_apply(
     elif axis == 1:
         if columns:
             raise NotImplementedError(
-                "func given to frame_apply cannot contain "
-                "an index relabeling when axis is 1"
+                f"Named aggregation is not supported when {axis=}."
             )
         klass = FrameColumnApply
 

pandas/core/shared_docs.py

@@ -50,7 +50,7 @@
 
 A passed user-defined-function will be passed a Series for evaluation.
 
-If `func` defines an index relabeling, `axis` must be `0` or `index`.
+If ``func`` defines an index relabeling, ``axis`` must be ``0`` or ``index``.
 {examples}"""
 
 _shared_docs["compare"] = """

pandas/tests/apply/test_frame_apply.py

@@ -1329,9 +1329,13 @@ def test_agg_reduce(axis, float_frame):
     expected = expected.T if axis in {1, "columns"} else expected
     tm.assert_frame_equal(result, expected)
 
-    msg = "func given to frame_apply cannot contain an index relabeling when axis is 1"
+
+def test_named_agg_reduce_axis1_raises(float_frame):
+    axis = 1
+    name1, name2 = float_frame.axes[0].unique()[:2].sort_values()
+    msg = f"Named aggregation is not supported when {axis=}."
     with pytest.raises(NotImplementedError, match=msg):
-        float_frame.agg(row1=(name1, "sum"), row2=(name2, "max"), axis=1)
+        float_frame.agg(row1=(name1, "sum"), row2=(name2, "max"), axis=axis)
 
 
 def test_nuiscance_columns():