Hey @TomNicholas, thanks for replying me back!

The problem is they chose to define how to hash a list (which is no hashable) by parsing it to a tuple

```python class frozenlist(list): def readonly(self, args, *kwargs): raise RuntimeError("Cannot modify ReadOnlyList")

# https://docs.python.org/3/library/pickle.html#object.__reduce__
#
# Like frozendict, implement __reduce__ and __setstate__ to handle pickling.
# Otherwise, __setstate__ will be called to restore the frozenlist, causing
# a RuntimeError because frozenlist is not mutable.

def __reduce__(self):
    return (frozenlist, (), list(self))

def __setstate__(self, state):
    self.__init__(state)

__setitem__ = __readonly__  # type: ignore[assignment]
__delitem__ = __readonly__
append = __readonly__
clear = __readonly__
extend = __readonly__
insert = __readonly__
pop = __readonly__
remove = __readonly__
reverse = __readonly__
sort = __readonly__  # type: ignore[assignment]

def __hash__(self):
    return hash(tuple(self))

```

Dataset class defines getitem by

python def __getitem__( self: T_Dataset, key: Mapping[Any, Any] | Hashable | Iterable[Hashable] ) -> T_Dataset | DataArray: """Access variables or coordinates of this dataset as a :py:class:`~xarray.DataArray` or a subset of variables or a indexed dataset. Indexing with a list of names will return a new ``Dataset`` object. """ if utils.is_dict_like(key): return self.isel(**key) if utils.hashable(key): return self._construct_dataarray(key) if utils.iterable_of_hashable(key): return self._copy_listed(key) raise ValueError(f"Unsupported key-type {type(key)}")

It tries to hash the key (and this frozenlist do this without any error).

I've checked out how pandas library deal with this task. They have a much more complex structure to deal with this problem (I haven't read all of the code below to understand):

```python def getitem(self, key): check_deprecated_indexers(key) key = lib.item_from_zerodim(key) key = com.apply_if_callable(key, self)

    if is_hashable(key) and not is_iterator(key):
        # is_iterator to exclude generator e.g. test_getitem_listlike
        # shortcut if the key is in columns
        is_mi = isinstance(self.columns, MultiIndex)
        # GH#45316 Return view if key is not duplicated
        # Only use drop_duplicates with duplicates for performance
        if not is_mi and (
            self.columns.is_unique
            and key in self.columns
            or key in self.columns.drop_duplicates(keep=False)
        ):
            return self._get_item_cache(key)

        elif is_mi and self.columns.is_unique and key in self.columns:
            return self._getitem_multilevel(key)
    # Do we have a slicer (on rows)?
    indexer = convert_to_index_sliceable(self, key)
    if indexer is not None:
        if isinstance(indexer, np.ndarray):
            indexer = lib.maybe_indices_to_slice(
                indexer.astype(np.intp, copy=False), len(self)
            )
            if isinstance(indexer, np.ndarray):
                # GH#43223 If we can not convert, use take
                return self.take(indexer, axis=0)
        # either we have a slice or we have a string that can be converted
        #  to a slice for partial-string date indexing
        return self._slice(indexer, axis=0)

    # Do we have a (boolean) DataFrame?
    if isinstance(key, DataFrame):
        return self.where(key)

    # Do we have a (boolean) 1d indexer?
    if com.is_bool_indexer(key):
        return self._getitem_bool_array(key)

    # We are left with two options: a single key, and a collection of keys,
    # We interpret tuples as collections only for non-MultiIndex
    is_single_key = isinstance(key, tuple) or not is_list_like(key)

    if is_single_key:
        if self.columns.nlevels > 1:
            return self._getitem_multilevel(key)
        indexer = self.columns.get_loc(key)
        if is_integer(indexer):
            indexer = [indexer]
    else:
        if is_iterator(key):
            key = list(key)
        indexer = self.columns._get_indexer_strict(key, "columns")[1]

    # take() does not accept boolean indexers
    if getattr(indexer, "dtype", None) == bool:
        indexer = np.where(indexer)[0]

    data = self._take_with_is_copy(indexer, axis=1)

    if is_single_key:
        # What does looking for a single key in a non-unique index return?
        # The behavior is inconsistent. It returns a Series, except when
        # - the key itself is repeated (test on data.shape, #9519), or
        # - we have a MultiIndex on columns (test on self.columns, #21309)
        if data.shape[1] == 1 and not isinstance(self.columns, MultiIndex):
            # GH#26490 using data[key] can cause RecursionError
            return data._get_item_cache(key)

    return data

```

This similar code, by the way, runs correctly

```python

%%

import pandas as pd data = pd.DataFrame({'a': [1], 'b': [2]}) variables = frozenlist(['a']) data[variables] ```

Now it is a matter of how xarray want to treat this issue. If you do want to change your way of doing this, I could try coding and request this PR trying pandas approach.

Anyway, I will report this issue back to Dagster for them to check this discussion too. Maybe they see their side is worth changing.

Let me know what is your view about this!

Links: https://github.com/pydata/xarray/blob/e1936a98059ae29da2861f58a7aff4a56302aac1/xarray/core/dataset.py#L1419 https://github.com/pandas-dev/pandas/blob/v1.5.1/pandas/core/frame.py#L473-L11983