Group by median fails with dask arrays with xarray==2026.9.0 #11651

# /// 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
import dask.array as da

xr.show_versions()


xr.DataArray(
    da.arange(8, chunks=(1,)).reshape((4, 2)),
    dims=("x", "y"), coords={"x": ['1', '2', '1', '2']}, name='').groupby("x").median()

                                                       
  File "python3.13/site-packages/xarray/core/_aggregations.py", line 8046, in median
    return self._flox_reduce(
           ~~~~~~~~~~~~~~~~~^
        func="median",
        ^^^^^^^^^^^^^^
    ...<4 lines>...
        **kwargs,
        ^^^^^^^^^
    )
    ^
  File "python3.13/site-packages/xarray/core/groupby.py", line 1157, in _flox_reduce
    result = xarray_reduce(
        obj.drop_vars(non_numeric.keys()),
    ...<5 lines>...
        **kwargs,
    )
  File "python3.13/site-packages/flox/xarray.py", line 416, in xarray_reduce
    actual = xr.apply_ufunc(
        wrapper,
    ...<27 lines>...
        },
    )
  File "python3.13/site-packages/xarray/computation/apply_ufunc.py", line 1279, in apply_ufunc
    return apply_dataset_vfunc(
        variables_vfunc,
    ...<7 lines>...
        on_missing_core_dim=on_missing_core_dim,
    )
  File "python3.13/site-packages/xarray/computation/apply_ufunc.py", line 527, in apply_dataset_vfunc
    result_vars = apply_dict_of_variables_vfunc(
        func,
    ...<4 lines>...
        on_missing_core_dim=on_missing_core_dim,
    )
  File "python3.13/site-packages/xarray/computation/apply_ufunc.py", line 452, in apply_dict_of_variables_vfunc
    result_vars[name] = func(*variable_args)
                        ~~~~^^^^^^^^^^^^^^^^
  File "python3.13/site-packages/xarray/computation/apply_ufunc.py", line 820, in apply_variable_ufunc
    result_data = func(*input_data)
  File "python3.13/site-packages/flox/xarray.py", line 373, in wrapper
    result, *groups = groupby_reduce(array, *by, func=func, **kwargs)
                      ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "python3.13/site-packages/flox/core.py", line 1196, in groupby_reduce
    result = reindex_(
             ~~~~~~~~^
        result,
        ^^^^^^^
    ...<3 lines>...
        array_type=ReindexArrayType.AUTO,  # just reindex the received array
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ).reshape(result.shape[:-1] + grp_shape)
    ^
  File "python3.13/site-packages/flox/reindex.py", line 213, in reindex_
    reindexed = reindex_numpy(array, from_, to, fill_value, new_dtype, axis)
  File "python3.13/site-packages/flox/reindex.py", line 93, in reindex_numpy
    idx = from_.get_indexer(to)
  File "python3.13/site-packages/pandas/core/indexes/base.py", line 3892, in get_indexer
    raise InvalidIndexError(self._requires_unique_msg)
pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects