BUG: Fix for fillna ignoring axis=1 parameter (issues #17399 #17409)

doc/source/whatsnew/v1.0.0.rst

@@ -233,7 +233,7 @@ Other
 - Trying to set the ``display.precision``, ``display.max_rows`` or ``display.max_columns`` using :meth:`set_option` to anything but a ``None`` or a positive int will raise a ``ValueError`` (:issue:`23348`)
 - Using :meth:`DataFrame.replace` with overlapping keys in a nested dictionary will no longer raise, now matching the behavior of a flat dictionary (:issue:`27660`)
 - :meth:`DataFrame.to_csv` and :meth:`Series.to_csv` now support dicts as ``compression`` argument with key ``'method'`` being the compression method and others as additional compression options when the compression method is ``'zip'``. (:issue:`26023`)
--
+- :meth:`DataFrame.fillna` when using axis=1 previously failed to replace NaN values (:issue:`17399` and :issue:`17409`)
 
 .. _whatsnew_1000.contributors:
 

pandas/core/generic.py

@@ -6274,7 +6274,8 @@ def fillna(
                     )
 
                 new_data = self._data.fillna(
-                    value=value, limit=limit, inplace=inplace, downcast=downcast
+                    value=value, limit=limit, inplace=inplace,
+                    downcast=downcast, axis: Axis=axis
                 )
 
             elif isinstance(value, (dict, ABCSeries)):

pandas/core/internals/blocks.py

@@ -81,6 +81,7 @@
 from pandas.core.nanops import nanpercentile
 
 from pandas.io.formats.printing import pprint_thing
+from pandas_typing import Axis
 
 
 class Block(PandasObject):
@@ -386,7 +387,7 @@ def apply(self, func, **kwargs):
 
         return result
 
-    def fillna(self, value, limit=None, inplace=False, downcast=None):
+    def fillna(self, value, limit=None, inplace=False, downcast=None, axis: Axis = 0):
         """ fillna on the block with the value. If we fail, then convert to
         ObjectBlock and try again
         """
@@ -398,13 +399,10 @@ def fillna(self, value, limit=None, inplace=False, downcast=None):
                 raise ValueError("Limit must be an integer")
             if limit < 1:
                 raise ValueError("Limit must be greater than 0")
-            mask[mask.cumsum(self.ndim - 1) > limit] = False
+            mask[mask.cumsum(int(axis == 0 and self.ndim > 1)) > limit] = False
 
         if not self._can_hold_na:
-            if inplace:
-                return self
-            else:
-                return self.copy()
+            return self if inplace else self.copy()
 
         if self._can_hold_element(value):
             # equivalent: _try_coerce_args(value) would not raise
@@ -1842,7 +1840,7 @@ def concat_same_type(self, to_concat, placement=None):
         placement = placement or slice(0, len(values), 1)
         return self.make_block_same_class(values, ndim=self.ndim, placement=placement)
 
-    def fillna(self, value, limit=None, inplace=False, downcast=None):
+    def fillna(self, value, limit=None, inplace=False, downcast=None, axis: Axis = 0):
         values = self.values if inplace else self.values.copy()
         values = values.fillna(value=value, limit=limit)
         return [
@@ -2406,15 +2404,15 @@ def concat_same_type(self, to_concat, placement=None):
             return ObjectBlock(values, ndim=self.ndim, placement=placement)
         return super().concat_same_type(to_concat, placement)
 
-    def fillna(self, value, limit=None, inplace=False, downcast=None):
+    def fillna(self, value, limit=None, inplace=False, downcast=None, axis: Axis = 0):
         # We support filling a DatetimeTZ with a `value` whose timezone
         # is different by coercing to object.
         if self._can_hold_element(value):
-            return super().fillna(value, limit, inplace, downcast)
+            return super().fillna(value, limit, inplace, downcast, axis)
 
         # different timezones, or a non-tz
         return self.astype(object).fillna(
-            value, limit=limit, inplace=inplace, downcast=downcast
+            value, limit=limit, inplace=inplace, downcast=downcast, axis=axis
         )
 
     def setitem(self, indexer, value):

pandas/tests/frame/test_missing.py

@@ -671,6 +671,21 @@ def test_fillna_columns(self):
         expected = df.astype(float).fillna(method="ffill", axis=1)
         assert_frame_equal(result, expected)
 
+    def test_fillna_rows(self):
+        #GH17399
+        df = pd.DataFrame({
+            "a": [1,2,3,4],
+            "b": [5,np.nan,7,8],
+            "c": [9,10,11,np.nan]})
+
+        expected = pd.DataFrame({
+            "a": [1,2,3,4],
+            "b": [5,6,7,8],
+            "c": [9,10,11,6]})
+
+        result = df.fillna(df.mean(axis=1))
+        tm.assert_frame_equal(result, expected)
+
     def test_fillna_invalid_method(self, float_frame):
         with pytest.raises(ValueError, match="ffil"):
             float_frame.fillna(method="ffil")