BUG : Allow axis = 'columns' with limit and no method to work on fillna. (#42856)

* solves issue #40989 by raising NotImplementedError and modifying docs

* Testing the NotImplementedError created to solve issue #40989

* pre-commit error fixed

* added modified generic.py

* added modified test_fillna.py

* reverted changes for test_frame.py

* added another test

* added into bug fixes in whatsnew

Author: GYvan

doc/source/whatsnew/v1.4.0.rst

@@ -303,7 +303,7 @@ Indexing
 
 Missing
 ^^^^^^^
--
+- Bug in :meth:`DataFrame.fillna` with limit and no method ignores axis='columns' or ``axis = 1`` (:issue:`40989`)
 -
 
 MultiIndex

pandas/core/generic.py

@@ -6312,10 +6312,21 @@ def fillna(
                 return result if not inplace else None
 
             elif not is_list_like(value):
-                new_data = self._mgr.fillna(
-                    value=value, limit=limit, inplace=inplace, downcast=downcast
-                )
+                if not self._mgr.is_single_block and axis == 1:
+
+                    result = self.T.fillna(value=value, limit=limit).T
+
+                    # need to downcast here because of all of the transposes
+                    result._mgr = result._mgr.downcast()
+
+                    new_data = result
+                else:
+
+                    new_data = self._mgr.fillna(
+                        value=value, limit=limit, inplace=inplace, downcast=downcast
+                    )
             elif isinstance(value, ABCDataFrame) and self.ndim == 2:
+
                 new_data = self.where(self.notna(), value)._data
             else:
                 raise ValueError(f"invalid fill value with a {type(value)}")

pandas/tests/frame/methods/test_fillna.py

@@ -574,6 +574,60 @@ def test_fillna_pos_args_deprecation(self):
         expected = DataFrame({"a": [1, 2, 3, 0]}, dtype=float)
         tm.assert_frame_equal(result, expected)
 
+    def test_fillna_with_columns_and_limit(self):
+        # GH40989
+        df = DataFrame(
+            [
+                [np.nan, 2, np.nan, 0],
+                [3, 4, np.nan, 1],
+                [np.nan, np.nan, np.nan, 5],
+                [np.nan, 3, np.nan, 4],
+            ],
+            columns=list("ABCD"),
+        )
+        result = df.fillna(axis=1, value=100, limit=1)
+        result2 = df.fillna(axis=1, value=100, limit=2)
+
+        expected = DataFrame(
+            {
+                "A": Series([100, 3, 100, 100], dtype="float64"),
+                "B": [2, 4, np.nan, 3],
+                "C": [np.nan, 100, np.nan, np.nan],
+                "D": Series([0, 1, 5, 4], dtype="float64"),
+            },
+            index=[0, 1, 2, 3],
+        )
+        expected2 = DataFrame(
+            {
+                "A": Series([100, 3, 100, 100], dtype="float64"),
+                "B": Series([2, 4, 100, 3], dtype="float64"),
+                "C": [100, 100, np.nan, 100],
+                "D": Series([0, 1, 5, 4], dtype="float64"),
+            },
+            index=[0, 1, 2, 3],
+        )
+
+        tm.assert_frame_equal(result, expected)
+        tm.assert_frame_equal(result2, expected2)
+
+    def test_fillna_inplace_with_columns_limit_and_value(self):
+        # GH40989
+        df = DataFrame(
+            [
+                [np.nan, 2, np.nan, 0],
+                [3, 4, np.nan, 1],
+                [np.nan, np.nan, np.nan, 5],
+                [np.nan, 3, np.nan, 4],
+            ],
+            columns=list("ABCD"),
+        )
+
+        expected = df.fillna(axis=1, value=100, limit=1)
+        assert expected is not df
+
+        df.fillna(axis=1, value=100, limit=1, inplace=True)
+        tm.assert_frame_equal(df, expected)
+
 
 def test_fillna_nonconsolidated_frame():
     # https://github.com/pandas-dev/pandas/issues/36495