BUG: reindex would throw when a categorical index was empty #16770

doc/source/whatsnew/v0.20.3.txt

@@ -42,6 +42,7 @@ Bug Fixes
 - Fixed compat with loading a ``DataFrame`` with a ``PeriodIndex``, from a ``format='fixed'`` HDFStore, in Python 3, that was written in Python 2 (:issue:`16781`)
 - Fixed a bug in failing to compute rolling computations of a column-MultiIndexed ``DataFrame`` (:issue:`16789`, :issue:`16825`)
 - Bug in a DataFrame/Series with a ``TimedeltaIndex`` when slice indexing (:issue:`16637`)
+- Handle reindexing an empty categorical index rather than throwing (:issue:`16770`)
 
 
 Conversion

pandas/core/indexes/category.py

@@ -419,7 +419,11 @@ def reindex(self, target, method=None, level=None, limit=None,
             raise ValueError("cannot reindex with a non-unique indexer")
 
         indexer, missing = self.get_indexer_non_unique(np.array(target))
-        new_target = self.take(indexer)
+
+        if len(self.codes):
+            new_target = self.take(indexer)
+        else:
+            new_target = target
 
         # filling in missing if needed
         if len(missing):
@@ -430,7 +434,8 @@ def reindex(self, target, method=None, level=None, limit=None,
                 result = Index(np.array(self), name=self.name)
                 new_target, indexer, _ = result._reindex_non_unique(
                     np.array(target))
-
+                # see GH 16819, indexer needs to be converted to correct type
+                indexer = np.array(indexer, dtype=np.int64)
             else:
 
                 codes = new_target.codes.copy()

pandas/tests/indexes/test_category.py

@@ -419,6 +419,14 @@ def test_reindex_dtype(self):
         tm.assert_numpy_array_equal(indexer,
                                     np.array([0, 3, 2], dtype=np.int64))
 
+    def test_reindex_empty_index(self):
+        # See GH16770
+        c = CategoricalIndex([])
+        res, indexer = c.reindex(['a', 'b'])
+        tm.assert_index_equal(res, Index(['a', 'b']), exact=True)
+        tm.assert_numpy_array_equal(indexer,
+                                    np.array([-1, -1], dtype=np.int64))
+
     def test_duplicates(self):
 
         idx = CategoricalIndex([0, 0, 0], name='foo')