BUG: Series.loc[[]] with non-unique MultiIndex #13691 (#37161)

Author: jbrockmendel

doc/source/whatsnew/v1.2.0.rst

@@ -400,6 +400,7 @@ Indexing
 - Bug in :meth:`DataFrame.sort_index` where parameter ascending passed as a list on a single level index gives wrong result. (:issue:`32334`)
 - Bug in :meth:`DataFrame.reset_index` was incorrectly raising a ``ValueError`` for input with a :class:`MultiIndex` with missing values in a level with ``Categorical`` dtype (:issue:`24206`)
 - Bug in indexing with boolean masks on datetime-like values sometimes returning a view instead of a copy (:issue:`36210`)
+- Bug in :meth:`Series.loc.__getitem__` with a non-unique :class:`MultiIndex` and an empty-list indexer (:issue:`13691`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -3404,6 +3404,10 @@ def _reindex_non_unique(self, target):
 
         """
         target = ensure_index(target)
+        if len(target) == 0:
+            # GH#13691
+            return self[:0], np.array([], dtype=np.intp), None
+
         indexer, missing = self.get_indexer_non_unique(target)
         check = indexer != -1
         new_labels = self.take(indexer[check])

pandas/tests/series/indexing/test_multiindex.py

@@ -49,3 +49,17 @@ def test_nat_multi_index(ix_data, exp_data):
 
     expected = pd.DataFrame(exp_data)
     tm.assert_frame_equal(result, expected)
+
+
+def test_loc_getitem_multiindex_nonunique_len_zero():
+    # GH#13691
+    mi = pd.MultiIndex.from_product([[0], [1, 1]])
+    ser = pd.Series(0, index=mi)
+
+    res = ser.loc[[]]
+
+    expected = ser[:0]
+    tm.assert_series_equal(res, expected)
+
+    res2 = ser.loc[ser.iloc[0:0]]
+    tm.assert_series_equal(res2, expected)