Backport PR #47774 on branch 1.4.x (BUG: Fix fillna on multi indexed Dataframe doesn't work) (#48216)

Backport PR #47774: BUG: Fix fillna on multi indexed Dataframe doesn't work

Co-authored-by: Xingrong Chen <56777910+xr-chen@users.noreply.github.com>

Author: simonjayhawkins

doc/source/whatsnew/v1.4.4.rst

@@ -14,6 +14,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed regression in :meth:`DataFrame.fillna` not working :class:`DataFrame` with :class:`MultiIndex` (:issue:`47649`)
 - Fixed regression in taking NULL :class:`objects` from a :class:`DataFrame` causing a segmentation violation. These NULL values are created by :meth:`numpy.empty_like` (:issue:`46848`)
 - Fixed regression in :func:`concat` materializing :class:`Index` during sorting even if :class:`Index` was already sorted (:issue:`47501`)
 - Fixed regression in calling bitwise numpy ufuncs (for example, ``np.bitwise_and``) on Index objects (:issue:`46769`)

pandas/core/generic.py

@@ -6482,9 +6482,14 @@ def fillna(
                     if k not in result:
                         continue
                     downcast_k = downcast if not is_dict else downcast.get(k)
-                    result.loc[:, k] = result[k].fillna(
-                        v, limit=limit, downcast=downcast_k
+                    # GH47649
+                    result.loc[:, k] = (
+                        result[k].fillna(v, limit=limit, downcast=downcast_k).values
                     )
+                    # TODO: result.loc[:, k] = result.loc[:, k].fillna(
+                    #     v, limit=limit, downcast=downcast_k
+                    # )
+                    # Revert when GH45751 is fixed
                 return result if not inplace else None
 
             elif not is_list_like(value):

pandas/tests/frame/methods/test_fillna.py

@@ -666,6 +666,34 @@ def test_inplace_dict_update_view(self, val):
         tm.assert_frame_equal(df, expected)
         tm.assert_frame_equal(result_view, expected)
 
+    def test_fillna_with_multi_index_frame(self):
+        # GH 47649
+        pdf = DataFrame(
+            {
+                ("x", "a"): [np.nan, 2.0, 3.0],
+                ("x", "b"): [1.0, 2.0, np.nan],
+                ("y", "c"): [1.0, 2.0, np.nan],
+            }
+        )
+        expected = DataFrame(
+            {
+                ("x", "a"): [-1.0, 2.0, 3.0],
+                ("x", "b"): [1.0, 2.0, -1.0],
+                ("y", "c"): [1.0, 2.0, np.nan],
+            }
+        )
+        tm.assert_frame_equal(pdf.fillna({"x": -1}), expected)
+        tm.assert_frame_equal(pdf.fillna({"x": -1, ("x", "b"): -2}), expected)
+
+        expected = DataFrame(
+            {
+                ("x", "a"): [-1.0, 2.0, 3.0],
+                ("x", "b"): [1.0, 2.0, -2.0],
+                ("y", "c"): [1.0, 2.0, np.nan],
+            }
+        )
+        tm.assert_frame_equal(pdf.fillna({("x", "b"): -2, "x": -1}), expected)
+
 
 def test_fillna_nonconsolidated_frame():
     # https://github.com/pandas-dev/pandas/issues/36495