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

Author: ante012

doc/source/whatsnew/v0.21.0.txt

@@ -314,7 +314,7 @@ Conversion
 - Fix :func:`DataFrame.memory_usage` to support PyPy. Objects on PyPy do not have a fixed size, so an approximation is used instead (:issue:`17228`)
 - Fixed the return type of ``IntervalIndex.is_non_overlapping_monotonic`` to be a Python ``bool`` for consistency with similar attributes/methods.  Previously returned a ``numpy.bool_``. (:issue:`17237`)
 - Bug in ``IntervalIndex.is_non_overlapping_monotonic`` when intervals are closed on both sides and overlap at a point (:issue:`16560`)
-
+- Bug in :func:`Series.fillna` returns frame when ``inplace=True`` and ``value`` is dict (:issue:`16156`)
 
 Indexing
 ^^^^^^^^

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')