versionadded; empty case

Author: chris-b1

doc/source/categorical.rst

@@ -653,6 +653,8 @@ The same applies to ``df.append(df_different)``.
 Unioning
 ~~~~~~~~
 
+.. versionadded:: 0.18.2
+
 If you want to combine categoricals that do not necessarily have
 the same categories, the `union_categorical` function will
 combine a list-like of categoricals. The new categories

pandas/tools/tests/test_concat.py

@@ -965,6 +965,9 @@ def test_union_categorical(self):
         with tm.assertRaises(TypeError):
             union_categoricals([s, s2])
 
+        with tm.assertRaises(ValueError):
+            union_categoricals([])
+
     def test_concat_bug_1719(self):
         ts1 = tm.makeTimeSeries()
         ts2 = tm.makeTimeSeries()[::2]

pandas/types/concat.py

@@ -223,13 +223,18 @@ def union_categoricals(to_union):
     TypeError
         If any of the categoricals are ordered or all do not
         have the same dtype
+    ValueError
+        Emmpty list of categoricals passed
     """
     from pandas import Index, Categorical
 
+    if len(to_union) == 0:
+        raise ValueError('No Categoricals to union')
+
+    first = to_union[0]
     if any(c.ordered for c in to_union):
         raise TypeError("Can only combine unordered Categoricals")
 
-    first = to_union[0]
     if not all(com.is_dtype_equal(c.categories.dtype, first.categories.dtype)
                for c in to_union):
         raise TypeError("dtype of categories must be the same")