BUG: Raise KeyError when indexing a Series with MultiIndex (#26155)

doc/source/whatsnew/v0.25.0.rst

@@ -327,6 +327,7 @@ Indexing
 ^^^^^^^^
 
 - Improved exception message when calling :meth:`DataFrame.iloc` with a list of non-numeric objects (:issue:`25753`).
+- Bug in :meth:`DataFrame.loc` and :meth:`Series.loc` where ``KeyError`` was not raised for a ``MultiIndex`` when the key was less than or equal to the number of levels in the :class:`MultiIndex` (:issue:`14885`).
 - Bug in which :meth:`DataFrame.append` produced an erroneous warning indicating that a ``KeyError`` will be thrown in the future when the data to be appended contains new columns (:issue:`22252`).
 -
 

pandas/core/indexing.py

@@ -944,6 +944,12 @@ def _handle_lowerdim_multi_index_axis0(self, tup):
         except TypeError:
             # slices are unhashable
             pass
+        except KeyError as ek:
+            # raise KeyError if number of indexers match
+            # else IndexingError will be raised
+            if (len(tup) <= self.obj.index.nlevels
+                    and len(tup) > self.obj.ndim):
+                raise ek
         except Exception as e1:
             if isinstance(tup[0], (slice, Index)):
                 raise IndexingError("Handle elsewhere")

pandas/tests/indexing/multiindex/test_getitem.py

@@ -85,7 +85,8 @@ def test_series_getitem_returns_scalar(
 @pytest.mark.parametrize('indexer,expected_error,expected_error_msg', [
     (lambda s: s.__getitem__((2000, 3, 4)), KeyError, r"^356L?$"),
     (lambda s: s[(2000, 3, 4)], KeyError, r"^356L?$"),
-    (lambda s: s.loc[(2000, 3, 4)], IndexingError, 'Too many indexers'),
+    (lambda s: s.loc[(2000, 3, 4)], KeyError, r"^356L?$"),
+    (lambda s: s.loc[(2000, 3, 4, 5)], IndexingError, 'Too many indexers'),
     (lambda s: s.__getitem__(len(s)), IndexError, 'index out of bounds'),
     (lambda s: s[len(s)], IndexError, 'index out of bounds'),
     (lambda s: s.iloc[len(s)], IndexError,

pandas/tests/indexing/multiindex/test_loc.py

@@ -6,6 +6,7 @@
 
 import pandas as pd
 from pandas import DataFrame, Index, MultiIndex, Series
+from pandas.core.indexing import IndexingError
 from pandas.util import testing as tm
 
 
@@ -130,6 +131,18 @@ def test_loc_multiindex(self):
             with pytest.raises(KeyError, match=r"^2L?$"):
                 mi_int.ix[2]
 
+    def test_loc_multiindex_too_many_dims(self):
+        # GH 14885
+        s = Series(range(8), index=MultiIndex.from_product(
+            [['a', 'b'], ['c', 'd'], ['e', 'f']]))
+
+        with pytest.raises(KeyError, match=r"^\('a', 'b'\)$"):
+            s.loc['a', 'b']
+        with pytest.raises(KeyError, match=r"^\('a', 'd', 'g'\)$"):
+            s.loc['a', 'd', 'g']
+        with pytest.raises(IndexingError, match='Too many indexers'):
+            s.loc['a', 'd', 'g', 'j']
+
     def test_loc_multiindex_indexer_none(self):
 
         # GH6788