Handle range on heterogeneously typed Dataset column

holoviews/core/data/interface.py

@@ -287,9 +287,12 @@ def range(cls, dataset, dimension):
             return np.NaN, np.NaN
         else:
             try:
+                assert column.dtype.kind not in 'SUO'
                 return (np.nanmin(column), np.nanmax(column))
-            except TypeError:
-                column.sort()
+            except (AssertionError, TypeError):
+                column = [v for v in util.python2sort(column) if v is not None]
+                if not len(column):
+                    return np.NaN, np.NaN
                 return column[0], column[-1]
 
     @classmethod

holoviews/core/data/pandas.py

@@ -141,6 +141,9 @@ def range(cls, columns, dimension):
                 column = column.sort(inplace=False)
             else:
                 column = column.sort_values()
+            column = column[~column.isin([None])]
+            if not len(column):
+                return np.NaN, np.NaN
             return column.iloc[0], column.iloc[-1]
         else:
             return (column.min(), column.max())

holoviews/core/util.py

@@ -722,7 +722,7 @@ def max_range(ranges):
                 values = [(v1.to_datetime64(), v2.to_datetime64()) for v1, v2 in values]
             arr = np.array(values)
             if arr.dtype.kind in 'OSU':
-                arr = np.sort([v for v in arr.flat if not is_nan(v)])
+                arr = list(python2sort([v for v in arr.flat if not is_nan(v) and v is not None]))
                 return arr[0], arr[-1]
             if arr.dtype.kind in 'M':
                 return arr[:, 0].min(), arr[:, 1].max()

tests/core/data/testdataset.py

@@ -473,6 +473,10 @@ def test_dataset_redim_with_alias_dframe(self):
         self.assertEqual(dataset.redim(**{'X-label':'X'}), dataset_redim)
         self.assertEqual(dataset.redim(**{'x':'X'}), dataset_redim)
 
+    def test_dataset_mixed_type_range(self):
+        ds = Dataset((['A', 'B', 'C', None],), 'A')
+        self.assertEqual(ds.range(0), ('A', 'C'))
+
     def test_dataset_sort_vdim_ht(self):
         dataset = Dataset({'x':self.xs, 'y':-self.ys},
                           kdims=['x'], vdims=['y'])