issues

What happened?

To parallelize a computation over a sparse array, I want to create chunks after stacking the array and dropping fill values. Such an array has a multiindex, which breaks the map_blocks method (with an unhelpful error). The AssertionError raised suggests to me that the method does not account for such an array having (coordinate) variables that are not dimensions or a scalar.

What did you expect to happen?

I expect the MCVE below to return its input unmodified.

Minimal Complete Verifiable Example

```Python import xarray as xr

a = xr.DataArray([[0, 1], [2, 3]], {"x": [0, 1], "y": [0, 1]}) a = a.stack({"n": ("x", "y")}) a = a.chunk({"n": 2}) a.map_blocks(lambda x: x) ```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.
• [X] Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

```Python File ~/tmp/bug/venv/lib/python3.11/site-packages/xarray/core/dataarray.py:5526, in DataArray.map_blocks(self, func, args, kwargs, template) 5428 """ 5429 Apply a function to each block of this DataArray. 5430 (...) 5522 month (time) int64 dask.array<chunksize=(24,), meta=np.ndarray> 5523 """ 5524 from xarray.core.parallel import map_blocks -> 5526 return map_blocks(func, self, args, kwargs, template)

File ~/tmp/bug/venv/lib/python3.11/site-packages/xarray/core/parallel.py:539, in map_blocks(func, obj, args, kwargs, template) 535 for chunk_tuple in itertools.product(*ichunk.values()): 536 # mapping from dimension name to chunk index 537 chunk_index = dict(zip(ichunk.keys(), chunk_tuple)) --> 539 blocked_args = [ 540 subset_dataset_to_block(graph, gname, arg, input_chunk_bounds, chunk_index) 541 if isxr 542 else arg 543 for isxr, arg in zip(is_xarray, npargs) 544 ] 546 # raise nice error messages in _wrapper 547 expected: ExpectedDict = { 548 # input chunk 0 along a dimension maps to output chunk 0 along the same dimension 549 # even if length of dimension is changed by the applied function (...) 563 }, 564 }

File ~/tmp/bug/venv/lib/python3.11/site-packages/xarray/core/parallel.py:540, in <listcomp>(.0) 535 for chunk_tuple in itertools.product(*ichunk.values()): 536 # mapping from dimension name to chunk index 537 chunk_index = dict(zip(ichunk.keys(), chunk_tuple)) 539 blocked_args = [ --> 540 subset_dataset_to_block(graph, gname, arg, input_chunk_bounds, chunk_index) 541 if isxr 542 else arg 543 for isxr, arg in zip(is_xarray, npargs) 544 ] 546 # raise nice error messages in _wrapper 547 expected: ExpectedDict = { 548 # input chunk 0 along a dimension maps to output chunk 0 along the same dimension 549 # even if length of dimension is changed by the applied function (...) 563 }, 564 }

File ~/tmp/bug/venv/lib/python3.11/site-packages/xarray/core/parallel.py:195, in subset_dataset_to_block(graph, gname, dataset, input_chunk_bounds, chunk_index) 190 graph[chunk_variable_task] = ( 191 tuple, 192 [variable.dims, chunk, variable.attrs], 193 ) 194 else: --> 195 assert name in dataset.dims or variable.ndim == 0 197 # non-dask array possibly with dimensions chunked on other variables 198 # index into variable appropriately 199 subsetter = { 200 dim: _get_chunk_slicer(dim, chunk_index, input_chunk_bounds) 201 for dim in variable.dims 202 }

AssertionError:

```

Anything else we need to know?

No response

Environment