API: .fillna will now raise a NotImplementedError when passed a DataFrame (GH8377)

doc/source/v0.15.0.txt

@@ -278,6 +278,8 @@ API changes
 - ``DataFrame.info()`` now ends its output with a newline character (:issue:`8114`)
 - add ``copy=True`` argument to ``pd.concat`` to enable pass thrue of complete blocks (:issue:`8252`)
 
+- ``.fillna`` will now raise a ``NotImplementedError`` when passed a ``DataFrame`` (:issue:`8377`)
+
 .. _whatsnew_0150.dt:
 
 .dt accessor
@@ -956,7 +958,6 @@ Bug Fixes
 - Bug with stacked barplots and NaNs (:issue:`8175`).
 
 
-
 - Bug in interpolation methods with the ``limit`` keyword when no values needed interpolating (:issue:`7173`).
 - Bug where ``col_space`` was ignored in ``DataFrame.to_string()`` when ``header=False`` (:issue:`8230`).
 - Bug with ``DatetimeIndex.asof`` incorrectly matching partial strings and returning the wrong date (:issue:`8245`).

pandas/core/generic.py

@@ -2266,6 +2266,7 @@ def fillna(self, value=None, method=None, axis=0, inplace=False,
         axis = self._get_axis_number(axis)
         method = com._clean_fill_method(method)
 
+        from pandas import DataFrame
         if value is None:
             if method is None:
                 raise ValueError('must specify a fill method or value')
@@ -2308,10 +2309,14 @@ def fillna(self, value=None, method=None, axis=0, inplace=False,
             if len(self._get_axis(axis)) == 0:
                 return self
 
-            if self.ndim == 1 and value is not None:
+            if self.ndim == 1:
                 if isinstance(value, (dict, com.ABCSeries)):
                     from pandas import Series
                     value = Series(value)
+                elif not com.is_list_like(value):
+                    pass
+                else:
+                    raise ValueError("invalid fill value with a %s" % type(value))
 
                 new_data = self._data.fillna(value=value,
                                              limit=limit,
@@ -2331,11 +2336,15 @@ def fillna(self, value=None, method=None, axis=0, inplace=False,
                     obj = result[k]
                     obj.fillna(v, limit=limit, inplace=True)
                 return result
-            else:
+            elif not com.is_list_like(value):
                 new_data = self._data.fillna(value=value,
                                              limit=limit,
                                              inplace=inplace,
                                              downcast=downcast)
+            elif isinstance(value, DataFrame) and self.ndim == 2:
+                raise NotImplementedError("can't use fillna with a DataFrame, use .where instead")
+            else:
+                raise ValueError("invalid fill value with a %s" % type(value))
 
         if inplace:
             self._update_inplace(new_data)

pandas/tests/test_frame.py

@@ -7650,6 +7650,10 @@ def test_fillna_invalid_value(self):
         self.assertRaises(TypeError, self.frame.fillna, [1, 2])
         # tuple
         self.assertRaises(TypeError, self.frame.fillna, (1, 2))
+        # frame
+        self.assertRaises(NotImplementedError, self.frame.fillna, self.frame)
+        # frame with series
+        self.assertRaises(ValueError, self.frame.iloc[:,0].fillna, self.frame)
 
     def test_replace_inplace(self):
         self.tsframe['A'][:5] = nan