BUG: Indexing MultiIndex with Series failed.

Previously, accessing elements of a MultiIndex-indexed DataFrame with a Series
failed. This changes that behavior so that it is possible to use a Series to
access elements from a MultiIndex-indexed DataFrame, just as one would use
a list.

Author: Ben Kandel

doc/source/whatsnew/v0.20.0.txt

@@ -297,7 +297,7 @@ Bug Fixes
 - Bug in ``pd.to_numeric()`` in which float and unsigned integer elements were being improperly casted (:issue:`14941`, :issue:`15005`)
 - Bug in ``pd.read_csv()`` in which the ``dialect`` parameter was not being verified before processing (:issue:`14898`)
 
-
+- Bug in ``DataFrame`` where indexing a ``MultiIndex`` using a ``Series`` failed (:issue:`14730`)
 
 - Bug in ``pd.read_msgpack()`` in which ``Series`` categoricals were being improperly processed (:issue:`14901`)
 - Bug in ``Series.ffill()`` with mixed dtypes containing tz-aware datetimes. (:issue:`14956`)

pandas/core/indexing.py

@@ -1461,6 +1461,9 @@ def _getitem_axis(self, key, axis=0):
             if isinstance(labels, MultiIndex):
                 if (not isinstance(key, tuple) and len(key) > 1 and
                         not isinstance(key[0], tuple)):
+                    if isinstance(key, ABCSeries):
+                        # GH 14730
+                        key = key.values.tolist()
                     key = tuple([key])
 
             # an iterable multi-selection

pandas/tests/indexes/test_multi.py

@@ -1,26 +1,29 @@
 # -*- coding: utf-8 -*-
 
-from datetime import timedelta
-from itertools import product
-import nose
 import re
 import warnings
 
-from pandas import (DataFrame, date_range, period_range, MultiIndex, Index,
-                    CategoricalIndex, compat)
-from pandas.core.common import PerformanceWarning, UnsortedIndexError
-from pandas.indexes.base import InvalidIndexError
-from pandas.compat import range, lrange, u, PY3, long, lzip
+from datetime import timedelta
+from itertools import product
+
+import nose
 
 import numpy as np
 
-from pandas.util.testing import (assert_almost_equal, assertRaises,
-                                 assertRaisesRegexp, assert_copy)
+import pandas as pd
+
+from pandas import (CategoricalIndex, DataFrame, Index, MultiIndex, Series,
+                    compat, date_range, period_range)
+from pandas.compat import PY3, long, lrange, lzip, range, u
+from pandas.core.common import PerformanceWarning, UnsortedIndexError
+from pandas.indexes.base import InvalidIndexError
+from pandas.lib import Timestamp
 
 import pandas.util.testing as tm
 
-import pandas as pd
-from pandas.lib import Timestamp
+from pandas.util.testing import (assertRaises, assertRaisesRegexp,
+                                 assert_almost_equal, assert_copy)
+
 
 from .common import Base
 
@@ -343,6 +346,19 @@ def test_set_levels_labels_names_bad_input(self):
         with tm.assertRaisesRegexp(TypeError, 'string'):
             self.index.set_names(names, level=0)
 
+    def test_series_index(self):
+        # GH14730
+        index = MultiIndex.from_product([[1, 2, 3], ['A', 'B', 'C']])
+        x = Series(index=index, data=range(9))
+        y = Series([1, 3])
+        expected = Series(
+            data=[0, 1, 2, 6, 7, 8],
+            index=MultiIndex.from_product([[1, 3], ['A', 'B', 'C']]))
+        actual_from_series = x.loc[y]
+        actual_from_list = x.loc[[1, 3]]
+        tm.assert_series_equal(expected, actual_from_list)
+        tm.assert_series_equal(expected, actual_from_series)
+
     def test_set_levels_categorical(self):
         # GH13854
         index = MultiIndex.from_arrays([list("xyzx"), [0, 1, 2, 3]])