Using .sel with a list of booleans, but for a boolean dtype dimension raises weird errors

[doronbehar]

What is your issue?

Take this MWE:

#!/usr/bin/env python

import xarray as xr
import numpy as np

# Define coordinates
float_coords = np.linspace(0.0, 1.0, 5)  # Float coordinates from 0.0 to 1.0
bool_coords = np.array([True, False])    # Boolean coordinates
# Create the Dataset
example_dataset = xr.Dataset(
    coords={
        "float_dim": float_coords,
        "bool_dim": bool_coords,
    }
)
loc = {
    "float_dim": 0,
    "bool_dim": True,
}
print(example_dataset.sel({
    k: [v]
    for k,v in loc.items()
}))

It fails with:

Traceback (most recent call last):
  File "/home/doron/repos/lab-ion-trap-simulations/./t.py", line 22, in <module>
    print(example_dataset.sel({"bool_dim": [ True ]}))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/xarray/core/dataset.py", line 3237, in sel
    result = self.isel(indexers=query_results.dim_indexers, drop=drop)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/xarray/core/dataset.py", line 3080, in isel
    indexes, index_variables = isel_indexes(self.xindexes, indexers)
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/xarray/core/indexes.py", line 1872, in isel_indexes
    return _apply_indexes_fast(indexes, indexers, "isel")
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/xarray/core/indexes.py", line 1827, in _apply_indexes_fast
    new_index = getattr(index, func)(index_args)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/xarray/core/indexes.py", line 743, in isel
    return self._replace(self.index[indxr])  # type: ignore[index]
                         ~~~~~~~~~~^^^^^^^
  File "/nix/store/r2g9s5hcmndxk6lwxwavpg3ga33sf18c-python3-3.12.7-env/lib/python3.12/site-packages/pandas/core/indexes/base.py", line 5416, in __getitem__
    result = getitem(key)
             ^^^^^^^^^^^^
IndexError: boolean index did not match indexed array along dimension 0; dimension is 2 but corresponding boolean dimension is 1

However, using:

print(example_dataset.sel({"bool_dim":  True }))

Doesn't fail, but it squeezes bool_dim to a 0 size array, and I don't want that to happen, since want to end up with a bool_dim of size 1, and I don't want to use .expand_dims because I have a non-dimension coordinate attached to that bool_dim and I lose that attachment information due to #4501 .

I managed to find the following workaround using isel:

print(example_dataset.isel({
    k: (example_dataset[k].values == v).nonzero()[0]
    for k,v in loc.items()
}))

But it is not very elegant of course :).

EDIT:

Perhaps this is a duplicate of #4892 , but there no error is mentioned.

[welcome]

Thanks for opening your first issue here at xarray! Be sure to follow the issue template!
If you have an idea for a solution, we would really welcome a Pull Request with proposed changes.
See the Contributing Guide for more.
It may take us a while to respond here, but we really value your contribution. Contributors like you help make xarray better.
Thank you!

[dcherian]

Yes can you add your nice example as a comment to #4892 please?