BUG: fillna returns frame when inplace=True if value is a dict (pandas-dev#16156)

Author: ante012

doc/source/whatsnew/v0.21.0.txt

@@ -391,3 +391,4 @@ Other
 - Bug in :func:`eval` where the ``inplace`` parameter was being incorrectly handled (:issue:`16732`)
 - Bug in ``.isin()`` in which checking membership in empty ``Series`` objects raised an error (:issue:`16991`)
 - Bug in :func:`unique` where checking a tuple of strings raised a ``TypeError`` (:issue:`17108`)
+- Bug in :func: `fillna` returns frame when ``inplace=True`` and ``value`` is dict (:issue:`16156`)

pandas/core/generic.py

@@ -4054,7 +4054,8 @@ def fillna(self, value=None, method=None, axis=None, inplace=False,
                         continue
                     obj = result[k]
                     obj.fillna(v, limit=limit, inplace=True, downcast=downcast)
-                return result
+                return result if not inplace else None
+                
             elif not is_list_like(value):
                 new_data = self._data.fillna(value=value, limit=limit,
                                              inplace=inplace,

pandas/tests/frame/test_missing.py

@@ -407,6 +407,9 @@ def test_fillna_inplace(self):
         df.fillna(value=0, inplace=True)
         tm.assert_frame_equal(df, expected)
 
+        expected = df.fillna(value={0 : 0}, inplace=True)
+        assert expected is None
+
         df[1][:4] = np.nan
         df[3][-4:] = np.nan
         expected = df.fillna(method='ffill')