BUG: fix sum over level failure with MultiIndex of length 1, close #1568

Author: wesm

RELEASE.rst

@@ -60,6 +60,7 @@ pandas 0.8.1
   - Fix DataFrame.rank error on integer data (#1589)
   - Selection of multiple SparseDataFrame columns by list in __getitem__ (#1585)
   - Override Index.tolist for compatibility with MultiIndex (#1576)
+  - Fix hierarchical summing bug with MultiIndex of length 1 (#1568)
 
 pandas 0.8.0
 ============

pandas/core/groupby.py

@@ -1083,7 +1083,7 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True):
 
     if (not any_callable and not all_in_columns
         and not any_arraylike and match_axis_length
-        and not level):
+        and level is None):
         keys = [com._asarray_tuplesafe(keys)]
 
     if isinstance(level, (tuple, list)):

pandas/tests/test_multilevel.py

@@ -1002,6 +1002,13 @@ def aggf(x):
 
             assert_frame_equal(leftside, rightside)
 
+    def test_stat_op_corner(self):
+        obj = Series([10.0], index=MultiIndex.from_tuples([(2, 3)]))
+
+        result = obj.sum(level=0)
+        expected = Series([10.0], index=[2])
+        assert_series_equal(result, expected)
+
     def test_frame_any_all_group(self):
         df = DataFrame({'data': [False, False, True, False, True, False, True]},
                        index=[['one', 'one', 'two', 'one', 'two', 'two', 'two'],