ENH:column-wise DataFrame.fillna

Author: proost

doc/source/whatsnew/v1.0.0.rst

@@ -227,7 +227,10 @@ Other enhancements
 - Added new writer for exporting Stata dta files in version 118, ``StataWriter118``.  This format supports exporting strings containing Unicode characters (:issue:`23573`)
 - :meth:`Series.map` now accepts ``collections.abc.Mapping`` subclasses as a mapper (:issue:`29733`)
 - The ``pandas.datetime`` class is now deprecated. Import from ``datetime`` instead (:issue:`30296`)
-
+- Added an experimental :attr:`~DataFrame.attrs` for storing global metadata about a dataset (:issue:`29062`)
+- :meth:`Timestamp.fromisocalendar` is now compatible with python 3.8 and above (:issue:`28115`)
+- :meth:`DataFrame.to_pickle` and :func:`read_pickle` now accept URL (:issue:`30163`)
+- :meth:`DataFrame.fillna` can fill NA values column-wise with a dictionary or :class:`Series` (:issue:`4514`)
 
 
 Build Changes

pandas/core/generic.py

@@ -6023,14 +6023,14 @@ def fillna(
                 )
 
             elif isinstance(value, (dict, ABCSeries)):
+                result = self if inplace else self.copy()
+
                 if axis == 1:
-                    raise NotImplementedError(
-                        "Currently only can fill "
-                        "with dict/Series column "
-                        "by column"
-                    )
+                    for label in self.columns:
+                        obj = result[label]
+                        obj.fillna(value, limit=limit, inplace=True, downcast=downcast)
+                    return result if not inplace else None
 
-                result = self if inplace else self.copy()
                 for k, v in value.items():
                     if k not in result:
                         continue

pandas/tests/frame/test_missing.py

@@ -604,9 +604,15 @@ def test_fillna_dict_series(self):
         expected = df.fillna(df.max().to_dict())
         tm.assert_frame_equal(result, expected)
 
-        # disable this for now
-        with pytest.raises(NotImplementedError, match="column by column"):
-            df.fillna(df.max(1), axis=1)
+        expected = DataFrame(
+            {
+                "a": [1.0, 1.0, 2.0, 3.0, 4.0],
+                "b": [1.0, 2.0, 3.0, 3.0, 4.0],
+                "c": [1.0, 1.0, 2.0, 3.0, 4.0],
+            }
+        )
+        result = df.fillna(df.max(1), axis=1)
+        tm.assert_frame_equal(expected, result)
 
     def test_fillna_dataframe(self):
         # GH 8377
@@ -983,3 +989,25 @@ def test_interp_time_inplace_axis(self, axis):
         result = expected.interpolate(axis=0, method="time")
         expected.interpolate(axis=0, method="time", inplace=True)
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "expected,fill_value",
+        [
+            (
+                DataFrame([["a", "a"], ["b", 4], [5, 6]], columns=list("AB")),
+                Series(["a", "b", "c"]),
+            ),
+            (
+                DataFrame([["a", "a"], [np.nan, 4], [5, 6]], columns=list("AB")),
+                {0: "a", 2: "c", 3: "d"},
+            ),
+        ],
+    )
+    def test_fillna_column_wise(self, expected, fill_value):
+        # GH 4514
+        df = DataFrame([[np.nan, np.nan], [np.nan, 4], [5, 6]], columns=list("AB"))
+        result = df.fillna(fill_value, axis=1)
+        tm.assert_frame_equal(expected, result)
+
+        df.fillna(fill_value, axis=1, inplace=True)
+        tm.assert_frame_equal(expected, df)