Fix: collapse dimension for single nested tuple MultiIndex key

xarray/core/indexes.py

@@ -9,6 +9,7 @@
 
 import numpy as np
 import pandas as pd
+from pandas.errors import InvalidIndexError
 
 from xarray.core import formatting, nputils, utils
 from xarray.core.coordinate_transform import CoordinateTransform
@@ -1367,7 +1368,14 @@ def sel(self, labels, method=None, tolerance=None) -> IndexSelResult:
 
             elif isinstance(label, tuple):
                 if _is_nested_tuple(label):
-                    indexer = self.index.get_locs(label)
+                    # When the tuple contains sub-tuples/lists/slices, it could be:
+                    # 1. A single key with tuple-valued levels (e.g., ((1, 1), 2))
+                    # 2. A multi-value selection (e.g., (1, slice(1, 2)))
+                    # Try get_loc first for case 1, fall back to get_locs for case 2
+                    try:
+                        indexer = self.index.get_loc(label)
+                    except (InvalidIndexError, KeyError):
+                        indexer = self.index.get_locs(label)
                 elif len(label) == self.index.nlevels:
                     indexer = self.index.get_loc(label)
                 else:

xarray/tests/test_indexes.py

@@ -559,6 +559,22 @@ def test_copy(self) -> None:
         assert actual.dim == expected.dim
         assert actual.level_coords_dtype == expected.level_coords_dtype
 
+    def test_sel_nested_tuple_key_collapses_dimension(self) -> None:
+        # Regression test for GH#11341
+        # When indexing with a single nested tuple key (where one level has
+        # tuple-valued entries), the dimension should collapse like scalar selection
+        nested_level_0 = pd.Index(
+            [(1, 1), (1, 1), (2, 2), (3, 3)], name="a", tupleize_cols=False
+        )
+        nested_level_1 = pd.Index([1, 2, 10, 20], name="b")
+        nested_mi = pd.MultiIndex.from_arrays([nested_level_0, nested_level_1])
+        index = PandasMultiIndex(nested_mi, "index")
+
+        # A single nested tuple key should produce a scalar indexer (int),
+        # not an array, collapsing the dimension
+        actual = index.sel({"index": ((1, 1), 2)})
+        assert isinstance(actual.dim_indexers["index"], (int, np.integer))
+
 
 class TestIndexes:
     @pytest.fixture