issues

What happened?

When opening a dataset containing a variable that has preferred chunks expressed along some dimension as a tuple of integers, xarray raises a TypeError.

What did you expect to happen?

I expected to open the dataset with its preferred chunks, as described in the documentation on preferred chunks within "How to add a new backend".

Minimal Complete Verifiable Example

```Python import xarray as xr

class PassThroughBackendEntrypoint(xr.backends.BackendEntrypoint): def open_dataset(self, dataset, *, drop_variables=None): return dataset

initial = xr.Dataset( { "data": xr.Variable( ("dim",), [0, 0], encoding={"preferred_chunks": {"dim": (1, 1)}} ) } ) final = xr.open_dataset(initial, engine=PassThroughBackendEntrypoint, chunks={}) ```

Relevant log output

```Python [Paths simplified.]

Traceback (most recent call last): File "<stdin>", line 1, in <module> File "...\xarray\backends\api.py", line 501, in open_dataset ds = _dataset_from_backend_dataset( File "...\xarray\backends\api.py", line 317, in _dataset_from_backend_dataset ds = _chunk_ds( File "...\xarray\backends\api.py", line 287, in _chunk_ds var_chunks = _get_chunk(var, chunks) File "...\xarray\core\dataset.py", line 409, in _get_chunk

_check_chunks_compatibility(var, output_chunks, preferred_chunks)

File "...\xarray\core\dataset.py", line 371, in _check_chunks_compatibility if any(s % preferred_chunks_dim for s in chunks_dim): File "...\xarray\core\dataset.py", line 371, in <genexpr> if any(s % preferred_chunks_dim for s in chunks_dim): TypeError: unsupported operand type(s) for %: 'int' and 'tuple' ```

Anything else we need to know?

The behavior exhibited above touches on the following related issues:

• The _check_chunks_compatibility function assumes that a dimension expresses its preferred chunks only as an integer, not a sequence of integers. In contrast, Dask will handle either within the previous_chunks argument to its normalize_chunks function.

• The examples in the documentation of "preferred_chunks" mappings, namely {“dim1”: 1000, “dim2”: 2000} and {“dim1”: [1000, 100], “dim2”: [2000, 2000, 2000]]}, have syntax errors: The quotation marks are curly instead of straight, and the second example has an extra closing bracket.

• After correcting the syntax errors, the lists in the second example lead to TypeError: unhashable type: 'list'. Dask raises the exception when it tries to test a mutable list for set membership, as in the following (with simplified paths):

  ```python

  dask.array.core.normalize_chunks([[1000, 100], [2000, 2000, 2000]], (1100, 6000)) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "...\dask\array\core.py", line 2900, in normalize_chunks chunks = tuple(c if c not in {None, -1} else s for c, s in zip(chunks, shape)) File "...\dask\array\core.py", line 2900, in <genexpr> chunks = tuple(c if c not in {None, -1} else s for c, s in zip(chunks, shape)) TypeError: unhashable type: 'list' ```

  If one omits the second argument (the shape) to that call, it succeeds. This may be a bug in Dask.

• The tests in xarray don't exercise behaviors related to preferred chunks.

[Edited for grammar.]

Environment

``` INSTALLED VERSIONS

commit: None python: 3.8.12 | packaged by conda-forge | (default, Oct 12 2021, 21:22:46) [MSC v.1916 64 bit (AMD64)] python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('English_United States', '1252') libhdf5: 1.12.1 libnetcdf: 4.8.1

xarray: 0.20.3.dev52+gd3b6aa6d pandas: 1.4.1 numpy: 1.21.5 scipy: 1.8.0 netCDF4: 1.5.8 pydap: installed h5netcdf: 0.13.1 h5py: 3.6.0 Nio: None zarr: 2.11.0 cftime: 1.5.2 nc_time_axis: 1.4.0 PseudoNetCDF: installed rasterio: 1.2.10 cfgrib: None iris: 3.2.0.post0 bottleneck: 1.3.2 dask: 2022.02.0 distributed: 2022.02.0 matplotlib: 3.5.1 cartopy: 0.20.2 seaborn: 0.11.2 numbagg: 0.2.1 fsspec: 2022.01.0 cupy: None pint: 0.18 sparse: 0.13.0 setuptools: 59.8.0 pip: 22.0.3 conda: None pytest: 7.0.1 IPython: None sphinx: None ```