Fixed metadata propagation in Dataframe.apply (issue #28283) (#44041)

Author: moha-rk

doc/source/whatsnew/v1.4.0.rst

@@ -609,6 +609,7 @@ Reshaping
 - Bug in :func:`concat` which ignored the ``sort`` parameter (:issue:`43375`)
 - Fixed bug in :func:`merge` with :class:`MultiIndex` as column index for the ``on`` argument returning an error when assigning a column internally (:issue:`43734`)
 - Bug in :func:`crosstab` would fail when inputs are lists or tuples (:issue:`44076`)
+- Fixed metadata propagation in :meth:`Dataframe.apply` method, consequently fixing the same issue for :meth:`Dataframe.transform`, :meth:`Dataframe.nunique` and :meth:`Dataframe.mode` (:issue:`28283`)
 
 Sparse
 ^^^^^^

pandas/core/frame.py

@@ -8872,7 +8872,7 @@ def apply(
             args=args,
             kwargs=kwargs,
         )
-        return op.apply()
+        return op.apply().__finalize__(self, method="apply")
 
     def applymap(
         self, func: PythonFuncType, na_action: str | None = None, **kwargs

pandas/tests/generic/test_finalize.py

@@ -233,7 +233,6 @@
     ),
     pytest.param(
         (pd.DataFrame, frame_data, operator.methodcaller("nunique")),
-        marks=not_implemented_mark,
     ),
     pytest.param(
         (pd.DataFrame, frame_data, operator.methodcaller("idxmin")),
@@ -245,6 +244,9 @@
     ),
     pytest.param(
         (pd.DataFrame, frame_data, operator.methodcaller("mode")),
+    ),
+    pytest.param(
+        (pd.Series, [0], operator.methodcaller("mode")),
         marks=not_implemented_mark,
     ),
     pytest.param(
@@ -467,12 +469,10 @@
             frame_mi_data,
             operator.methodcaller("transform", lambda x: x - x.min()),
         ),
-        marks=not_implemented_mark,
     ),
     (pd.Series, ([1],), operator.methodcaller("apply", lambda x: x)),
     pytest.param(
         (pd.DataFrame, frame_mi_data, operator.methodcaller("apply", lambda x: x)),
-        marks=not_implemented_mark,
     ),
     # Cumulative reductions
     (pd.Series, ([1],), operator.methodcaller("cumsum")),