issue_comments

Based on your suggestion above I tried this single line fix which resolved my issue: https://github.com/pydata/xarray/pull/7150

However I'm not sure if this is the correct approach, since I'm not all to deeply familiar with the indexing model.

Looks like the backend logic needs some updates to make it compatible with the new xarray data model with explicit indexes (i.e., possible indexed coordinates with name != dimension like for multi-index levels now), e.g., here:

https://github.com/pydata/xarray/blob/8eea8bb67bad0b5ac367c082125dd2b2519d4f52/xarray/backends/api.py#L234-L241

Here is the full stacktrace:

```python

ValueError Traceback (most recent call last) Cell In [12], line 7 ----> 7 loaded = xr.open_dataset("multiindex.nc", engine="netcdf4-multiindex", handle_multiindex=True) 8 print(loaded)

File ~/.local/share/virtualenvs/test-oePfdNug/lib/python3.8/site-packages/xarray/backends/api.py:537, in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, inline_array, backend_kwargs, kwargs) 530 overwrite_encoded_chunks = kwargs.pop("overwrite_encoded_chunks", None) 531 backend_ds = backend.open_dataset( 532 filename_or_obj, 533 drop_variables=drop_variables, 534 decoders, 535 kwargs, 536 ) --> 537 ds = _dataset_from_backend_dataset( 538 backend_ds, 539 filename_or_obj, 540 engine, 541 chunks, 542 cache, 543 overwrite_encoded_chunks, 544 inline_array, 545 drop_variables=drop_variables, 546 decoders, 547 **kwargs, 548 ) 549 return ds

File ~/.local/share/virtualenvs/test-oePfdNug/lib/python3.8/site-packages/xarray/backends/api.py:345, in _dataset_from_backend_dataset(backend_ds, filename_or_obj, engine, chunks, cache, overwrite_encoded_chunks, inline_array, **extra_tokens) 340 if not isinstance(chunks, (int, dict)) and chunks not in {None, "auto"}: 341 raise ValueError( 342 f"chunks must be an int, dict, 'auto', or None. Instead found {chunks}." 343 ) --> 345 _protect_dataset_variables_inplace(backend_ds, cache) 346 if chunks is None: 347 ds = backend_ds

File ~/.local/share/virtualenvs/test-oePfdNug/lib/python3.8/site-packages/xarray/backends/api.py:239, in _protect_dataset_variables_inplace(dataset, cache) 237 if cache: 238 data = indexing.MemoryCachedArray(data) --> 239 variable.data = data

File ~/.local/share/virtualenvs/test-oePfdNug/lib/python3.8/site-packages/xarray/core/variable.py:2795, in IndexVariable.data(self, data) 2793 @Variable.data.setter # type: ignore[attr-defined] 2794 def data(self, data): -> 2795 raise ValueError( 2796 f"Cannot assign to the .data attribute of dimension coordinate a.k.a IndexVariable {self.name!r}. " 2797 f"Please use DataArray.assign_coords, Dataset.assign_coords or Dataset.assign as appropriate." 2798 )

ValueError: Cannot assign to the .data attribute of dimension coordinate a.k.a IndexVariable 'measurement'. Please use DataArray.assign_coords, Dataset.assign_coords or Dataset.assign as appropriate. ```

I can see this type of decoding breaking some assumption in the file reading process. A full traceback would help identify where.

I think the real solution is actually #4490, so you could explicitly provide a coder.

Hi @lukasbindreiter, could you add the whole error traceback please?