BUG: .loc with MultiIndex with tuple in level GH#27591

doc/source/whatsnew/v1.4.0.rst

@@ -222,6 +222,7 @@ Interval
 Indexing
 ^^^^^^^^
 - Bug in :meth:`DataFrame.truncate` and :meth:`Series.truncate` when the object's Index has a length greater than one but only one unique value (:issue:`42365`)
+- Bug in :meth:`Series.loc` and :meth:`DataFrame.loc` with a :class:`MultiIndex` when indexing with a tuple in which one of the levels is also a tuple (:issue:`27591`)
 - Bug in :meth:`Series.loc` when with a :class:`MultiIndex` whose first level contains only ``np.nan`` values (:issue:`42055`)
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`DatetimeIndex` when passing a string, the return type depended on whether the index was monotonic (:issue:`24892`)
 - Bug in indexing on a :class:`MultiIndex` failing to drop scalar levels when the indexer is a tuple containing a datetime-like string (:issue:`42476`)

pandas/core/indexes/multi.py

@@ -3281,35 +3281,62 @@ def _update_indexer(idxr: Index, indexer: Index) -> Index:
                 # are or'd)
 
                 indexers: Int64Index | None = None
-                for x in k:
-                    try:
-                        # Argument "indexer" to "_get_level_indexer" of "MultiIndex"
-                        # has incompatible type "Index"; expected "Optional[Int64Index]"
-                        item_lvl_indexer = self._get_level_indexer(
-                            x, level=i, indexer=indexer  # type: ignore[arg-type]
-                        )
-                    except KeyError:
-                        # ignore not founds; see discussion in GH#39424
-                        warnings.warn(
-                            "The behavior of indexing on a MultiIndex with a nested "
-                            "sequence of labels is deprecated and will change in a "
-                            "future version. `series.loc[label, sequence]` will "
-                            "raise if any members of 'sequence' or not present in "
-                            "the index's second level. To retain the old behavior, "
-                            "use `series.index.isin(sequence, level=1)`",
-                            # TODO: how to opt in to the future behavior?
-                            # TODO: how to handle IntervalIndex level? (no test cases)
-                            FutureWarning,
-                            stacklevel=7,
-                        )
-                        continue
-                    else:
-                        idxrs = _convert_to_indexer(item_lvl_indexer)
 
-                        if indexers is None:
-                            indexers = idxrs
+                # GH#27591 check if this is a single tuple key in the level
+                try:
+                    # Argument "indexer" to "_get_level_indexer" of "MultiIndex"
+                    # has incompatible type "Index"; expected "Optional[Int64Index]"
+                    lev_loc = self._get_level_indexer(
+                        k, level=i, indexer=indexer  # type: ignore[arg-type]
+                    )
+                except (InvalidIndexError, TypeError, KeyError) as err:
+                    # InvalidIndexError e.g. non-hashable, fall back to treating
+                    #  this as a sequence of labels
+                    # KeyError it can be ambiguous if this is a label or sequence
+                    #  of labels
+                    #  github.com/pandas-dev/pandas/issues/39424#issuecomment-871626708
+                    for x in k:
+                        if not is_hashable(x):
+                            # e.g. slice
+                            raise err
+                        try:
+                            # Argument "indexer" to "_get_level_indexer" of "MultiIndex"
+                            # has incompatible type "Index"; expected
+                            # "Optional[Int64Index]"
+                            item_lvl_indexer = self._get_level_indexer(
+                                x, level=i, indexer=indexer  # type: ignore[arg-type]
+                            )
+                        except KeyError:
+                            # ignore not founds; see discussion in GH#39424
+                            warnings.warn(
+                                "The behavior of indexing on a MultiIndex with a "
+                                "nested sequence of labels is deprecated and will "
+                                "change in a future version. "
+                                "`series.loc[label, sequence]` will raise if any "
+                                "members of 'sequence' or not present in "
+                                "the index's second level. To retain the old "
+                                "behavior, use `series.index.isin(sequence, level=1)`",
+                                # TODO: how to opt in to the future behavior?
+                                # TODO: how to handle IntervalIndex level?
+                                #  (no test cases)
+                                FutureWarning,
+                                stacklevel=7,
+                            )
+                            continue
                         else:
-                            indexers = indexers.union(idxrs, sort=False)
+                            idxrs = _convert_to_indexer(item_lvl_indexer)
+
+                            if indexers is None:
+                                indexers = idxrs
+                            else:
+                                indexers = indexers.union(idxrs, sort=False)
+
+                else:
+                    idxrs = _convert_to_indexer(lev_loc)
+                    if indexers is None:
+                        indexers = idxrs
+                    else:
+                        indexers = indexers.union(idxrs, sort=False)
 
                 if indexers is not None:
                     indexer = _update_indexer(indexers, indexer=indexer)

pandas/tests/indexing/test_loc.py

@@ -2541,6 +2541,36 @@ def test_loc_with_period_index_indexer():
     tm.assert_frame_equal(df, df.loc[list(idx)])
 
 
+def test_loc_getitem_multiindex_tuple_level():
+    # GH#27591
+    lev1 = ["a", "b", "c"]
+    lev2 = [(0, 1), (1, 0)]
+    lev3 = [0, 1]
+    cols = MultiIndex.from_product([lev1, lev2, lev3], names=["x", "y", "z"])
+    df = DataFrame(6, index=range(5), columns=cols)
+
+    # the lev2[0] here should be treated as a single label, not as a sequence
+    #  of labels
+    result = df.loc[:, (lev1[0], lev2[0], lev3[0])]
+
+    # TODO: i think this actually should drop levels
+    expected = df.iloc[:, :1]
+    tm.assert_frame_equal(result, expected)
+
+    alt = df.xs((lev1[0], lev2[0], lev3[0]), level=[0, 1, 2], axis=1)
+    tm.assert_frame_equal(alt, expected)
+
+    # same thing on a Series
+    ser = df.iloc[0]
+    expected2 = ser.iloc[:1]
+
+    alt2 = ser.xs((lev1[0], lev2[0], lev3[0]), level=[0, 1, 2], axis=0)
+    tm.assert_series_equal(alt2, expected2)
+
+    result2 = ser.loc[lev1[0], lev2[0], lev3[0]]
+    assert result2 == 6
+
+
 class TestLocSeries:
     @pytest.mark.parametrize("val,expected", [(2 ** 63 - 1, 3), (2 ** 63, 4)])
     def test_loc_uint64(self, val, expected):