add is_scalar check and improve error msg

Author: reidy-p

pandas/core/categorical.py

@@ -1680,7 +1680,7 @@ def fillna(self, value=None, method=None, limit=None):
                 values[indexer] = values_codes[values_codes != -1]
 
             # If value is not a dict or Series it should be a scalar
-            else:
+            elif is_scalar(value):
                 if not isna(value) and value not in self.categories:
                     raise ValueError("fill value must be in categories")
 
@@ -1692,6 +1692,11 @@ def fillna(self, value=None, method=None, limit=None):
                     else:
                         values[mask] = self.categories.get_loc(value)
 
+            else:
+                raise TypeError('"value" parameter must be a scalar, dict '
+                                'or Series, but you passed a '
+                                '"{0}"'.format(type(value).__name__))
+
         return self._constructor(values, categories=self.categories,
                                  ordered=self.ordered, fastpath=True)
 

pandas/core/generic.py

@@ -4304,8 +4304,9 @@ def fillna(self, value=None, method=None, axis=None, inplace=False,
                 elif not is_list_like(value):
                     pass
                 else:
-                    raise ValueError("invalid fill value with a %s" %
-                                     type(value))
+                    raise TypeError('"value" parameter must be a scalar, dict '
+                                    'or Series, but you passed a '
+                                    '"{0}"'.format(type(value).__name__))
 
                 new_data = self._data.fillna(value=value, limit=limit,
                                              inplace=inplace,

pandas/tests/frame/test_missing.py

@@ -564,7 +564,7 @@ def test_fillna_invalid_value(self):
         # tuple
         pytest.raises(TypeError, self.frame.fillna, (1, 2))
         # frame with series
-        pytest.raises(ValueError, self.frame.iloc[:, 0].fillna, self.frame)
+        pytest.raises(TypeError, self.frame.iloc[:, 0].fillna, self.frame)
 
     def test_fillna_col_reordering(self):
         cols = ["COL." + str(i) for i in range(5, 0, -1)]

pandas/tests/series/test_missing.py

@@ -408,6 +408,11 @@ def test_fillna_categorical_raise(self):
                                     'dict, but you passed a "tuple"'):
             s.fillna(('a', 'b'))
 
+        with tm.assert_raises_regex(TypeError,
+                                    '"value" parameter must be a scalar, dict '
+                                    'or Series, but you passed a "DataFrame"'):
+            s.fillna(DataFrame({1: ['a'], 3: ['b']}))
+
     def test_fillna_nat(self):
         series = Series([0, 1, 2, iNaT], dtype='M8[ns]')