BUG: Regresssion in handling of empty Series as indexers to Series (GH5877)

doc/source/release.rst

@@ -81,6 +81,7 @@ Bug Fixes
   - Bug in isnull handling ``NaT`` in an object array (:issue:`5443`)
   - Bug in ``to_datetime`` when passed a ``np.nan`` or integer datelike and a format string (:issue:`5863`)
   - Bug in groupby dtype conversion with datetimelike (:issue:`5869`)
+  - Regresssion in handling of empty Series as indexers to Series  (:issue:`5877`)
 
 pandas 0.13.0
 -------------

pandas/core/common.py

@@ -1660,7 +1660,7 @@ def _is_bool_indexer(key):
         if key.dtype == np.object_:
             key = np.asarray(_values_from_object(key))
 
-            if len(key) and not lib.is_bool_array(key):
+            if not lib.is_bool_array(key):
                 if isnull(key).any():
                     raise ValueError('cannot index with vector containing '
                                      'NA / NaN values')

pandas/tests/test_series.py

@@ -16,6 +16,7 @@
 from pandas import (Index, Series, DataFrame, isnull, notnull,
                     bdate_range, date_range, _np_version_under1p7)
 from pandas.core.index import MultiIndex
+from pandas.core.indexing import IndexingError
 from pandas.tseries.index import Timestamp, DatetimeIndex
 import pandas.core.config as cf
 import pandas.lib as lib
@@ -795,6 +796,28 @@ def test_getitem_boolean_empty(self):
         self.assertEqual(s.index.name, 'index_name')
         self.assertEqual(s.dtype, np.int64)
 
+        # GH5877
+        # indexing with empty series
+        s = Series(['A', 'B'])
+        expected = Series(np.nan,index=['C'],dtype=object)
+        result = s[Series(['C'], dtype=object)]
+        assert_series_equal(result, expected)
+
+        s = Series(['A', 'B'])
+        expected = Series(dtype=object)
+        result = s[Series([], dtype=object)]
+        assert_series_equal(result, expected)
+
+        # invalid because of the boolean indexer
+        # that's empty or not-aligned
+        def f():
+            s[Series([], dtype=bool)]
+        self.assertRaises(IndexingError, f)
+
+        def f():
+            s[Series([True], dtype=bool)]
+        self.assertRaises(IndexingError, f)
+
     def test_getitem_generator(self):
         gen = (x > 0 for x in self.series)
         result = self.series[gen]