BUG: df.fillna ignores axis when df is single block (#47714)

* Update generic.py

* Update test_fillna.py

* Update v1.5.0.rst

* fix array format

* fix what's new description

* fix format

* Update test_fillna.py

Author: GYHHAHA

doc/source/whatsnew/v1.5.0.rst

@@ -914,6 +914,7 @@ Missing
 - Bug in :meth:`Series.fillna` and :meth:`DataFrame.fillna` with :class:`IntervalDtype` and incompatible value raising instead of casting to a common (usually object) dtype (:issue:`45796`)
 - Bug in :meth:`DataFrame.interpolate` with object-dtype column not returning a copy with ``inplace=False`` (:issue:`45791`)
 - Bug in :meth:`DataFrame.dropna` allows to set both ``how`` and ``thresh`` incompatible arguments (:issue:`46575`)
+- Bug in :meth:`DataFrame.fillna` ignored ``axis`` when :class:`DataFrame` is single block (:issue:`47713`)
 
 MultiIndex
 ^^^^^^^^^^

pandas/core/generic.py

@@ -6679,7 +6679,7 @@ def fillna(
                 return result if not inplace else None
 
             elif not is_list_like(value):
-                if not self._mgr.is_single_block and axis == 1:
+                if axis == 1:
 
                     result = self.T.fillna(value=value, limit=limit).T
 

pandas/tests/frame/methods/test_fillna.py

@@ -685,6 +685,29 @@ def test_inplace_dict_update_view(self, val):
         tm.assert_frame_equal(df, expected)
         tm.assert_frame_equal(result_view, expected)
 
+    def test_single_block_df_with_horizontal_axis(self):
+        # GH 47713
+        df = DataFrame(
+            {
+                "col1": [5, 0, np.nan, 10, np.nan],
+                "col2": [7, np.nan, np.nan, 5, 3],
+                "col3": [12, np.nan, 1, 2, 0],
+                "col4": [np.nan, 1, 1, np.nan, 18],
+            }
+        )
+        result = df.fillna(50, limit=1, axis=1)
+        expected = DataFrame(
+            [
+                [5.0, 7.0, 12.0, 50.0],
+                [0.0, 50.0, np.nan, 1.0],
+                [50.0, np.nan, 1.0, 1.0],
+                [10.0, 5.0, 2.0, 50.0],
+                [50.0, 3.0, 0.0, 18.0],
+            ],
+            columns=["col1", "col2", "col3", "col4"],
+        )
+        tm.assert_frame_equal(result, expected)
+
 
 def test_fillna_nonconsolidated_frame():
     # https://github.com/pandas-dev/pandas/issues/36495