Differing behaviour between `sel` and `drop_sel` for MultiIndexes

[owena11]

What happened?

There appears to be differing behaviour between sel and drop_sel for MultiIndexes where the index contains a mix of types (i.e str, int). With drop_sel raising and error and sel behaving as expected.

What did you expect to happen?

The DataArray to be returned with the element matching the index dropped.

Minimal Complete Verifiable Example

# /// 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()
# your reproducer code ...
import numpy as np

# Create a DataArray with a Multiindex with mixed types.
arr = xr.DataArray(
    np.arange(6).reshape(2, 3),
    coords=[("x", ["a", "b"]), ("y", [0, 1, 2])],
)
stacked = arr.stack(z=("x", "y"))


# Select a single item using the multiindex
selected_stack = stacked.sel(z=("a",0))

# Try and delete an item using the multiindex
dropped_stack = stacked.drop_sel(z=("a",0))

Steps to reproduce

A run of the MCVE (uv run issue.py)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/xarray/core/dataset.py:6086: PerformanceWarning: dropping on a non-lexsorted multi-index without a level parameter may impact performance.
  new_index = index.drop(labels_for_dim, errors=errors)

Traceback (most recent call last):
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/base.py", line 3812, in get_loc
    return self._engine.get_loc(casted_key)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "pandas/_libs/index.pyx", line 167, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 196, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 7088, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 7096, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: np.str_('0')

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/data/users/aled.owen/repos/ao-scratch_worktrees/main/scripts/xarray_drop_sel.py", line 29, in <module>
    dropped_stack = stacked.drop_sel(z=("a",0))
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/xarray/core/dataarray.py", line 3364, in drop_sel
    ds = self._to_temp_dataset().drop_sel(labels, errors=errors)
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/xarray/core/dataset.py", line 6086, in drop_sel
    new_index = index.drop(labels_for_dim, errors=errors)
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/multi.py", line 2438, in drop
    loc = self.get_loc(level_codes)
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/multi.py", line 3059, in get_loc
    loc = self._get_level_indexer(key, level=0)
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/multi.py", line 3410, in _get_level_indexer
    idx = self._get_loc_single_level_index(level_index, key)
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/multi.py", line 2999, in _get_loc_single_level_index
    return level_index.get_loc(key)
           ~~~~~~~~~~~~~~~~~~~^^^^^
  File "/data/users/aled.owen/.cache/uv/environments-v2/xarray-drop-sel-7f718ae04b721fc0/lib/python3.13/site-packages/pandas/core/indexes/base.py", line 3819, in get_loc
    raise KeyError(key) from err
KeyError: np.str_('0')

Anything else we need to know?

Looks to be linked to the array conversion here where this changes the type trying to be index:

xarray/xarray/core/dataset.py

Line 6079 in b5e4b0e

labels_for_dim = np.asarray(labels_for_dim)

Need to look usage of pandas.Index.drop and pandas.MultiIndex.drop to understand implications of this conversion before proposing a fix.

Environment

Details

INSTALLED VERSIONS

commit: None
python: 3.13.5 | packaged by conda-forge | (main, Jun 16 2025, 08:27:50) [GCC 13.3.0]
python-bits: 64
OS: Linux
OS-release: 5.14.0-570.46.1.el9_6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: ('en_GB', 'UTF-8')
libhdf5: 1.14.6
libnetcdf: 4.9.3

xarray: 2025.10.2.dev13+gb5e4b0e02
pandas: 2.3.3
numpy: 2.3.4
scipy: 1.16.2
netCDF4: 1.7.3
pydap: 3.5.8
h5netcdf: 1.7.2
h5py: 3.15.1
zarr: 3.1.3
cftime: 1.6.5
nc_time_axis: 1.4.1
iris: None
bottleneck: 1.6.0
dask: 2025.10.0
distributed: 2025.10.0
matplotlib: 3.10.7
cartopy: 0.25.0
seaborn: 0.13.2
numbagg: 0.9.3
fsspec: 2025.9.0
cupy: None
pint: None
sparse: 0.17.0
flox: 0.10.7
numpy_groupies: 0.11.3
setuptools: None
pip: None
conda: None
pytest: None
mypy: None
IPython: None
sphinx: None

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