A single nested tuple MultiIndex key is located correctly but preserves the dimension #11341

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
xr.show_versions()

import numpy as np
import pandas as pd
from xarray import Coordinates

simple_level_0 = pd.Index([1, 1, 2, 3], name="a")
simple_level_1 = pd.Index([1, 2, 10, 20], name="b")
simple_mi = pd.MultiIndex.from_arrays([simple_level_0, simple_level_1])

simple = xr.DataArray(
    np.arange(4),
    dims=("index",),
    coords={**Coordinates.from_pandas_multiindex(simple_mi, "index")},
)

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])

nested = xr.DataArray(
    np.arange(4),
    dims=("index",),
    coords={**Coordinates.from_pandas_multiindex(nested_mi, "index")},
)

simple_result = simple.sel(index=(1, 2))
nested_result = nested.sel(index=((1, 1), 2))

print("simple_result:", simple_result)
print("simple dims:", simple_result.dims, "shape:", simple_result.shape)
print()

print("nested_result:", nested_result)
print("nested dims:", nested_result.dims, "shape:", nested_result.shape)
print()

print("simple pandas get_loc:", simple.indexes["index"].get_loc((1, 2)))
print("nested pandas get_loc:", nested.indexes["index"].get_loc(((1, 1), 2)))
print()

simple_result: <xarray.DataArray ()> Size: 8B
array(1)
Coordinates:
    index    object 8B (np.int64(1), np.int64(2))
    a        int64 8B 1
    b        int64 8B 2
simple dims: () shape: ()

nested_result: <xarray.DataArray (index: 1)> Size: 8B
array([1])
Coordinates:
  * index    (index) object 8B MultiIndex
  * a        (index) object 8B (1, 1)
  * b        (index) int64 8B 2
nested dims: ('index',) shape: (1,)

simple pandas get_loc: 1
nested pandas get_loc: 1