def __getitem__(self, key) -> "ConcatenatableArray":
        """Some cases of concat require supporting expanding dims by dimensions of size 1"""
        # see https://data-apis.org/array-api/2022.12/API_specification/indexing.html#multi-axis-indexing
        arr = self._array
        for axis, indexer_1d in enumerate(key):
            if indexer_1d is None:
                arr = np.expand_dims(arr, axis)
            elif indexer_1d is Ellipsis:
                pass
            else:
                raise UnexpectedDataAccess("Tried accessing data.")
        return ConcatenatableArray(arr)
Some cases of concat require supporting expanding dims by dimensions of size 1
xarray/tests/arrays.py