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 ```

• [X] Closes #6333
• [X] Tests added
• [X] User visible changes (including notable bug fixes) are documented in whats-new.rst

Adding the import resolves the following exception that sphinx-build raises when one builds only "doc/internals/how-to-add-new-backend.rst":

``` NameError Traceback (most recent call last) Input In [1], in <module> ----> 1 var = xr.Variable( 2 dims=("x",), data=np.arange(10.0), attrs={"scale_factor": 10, "add_offset": 2} 3 )

NameError: name 'xr' is not defined ```

While in the file, I revised headings to have only their first letter capitalized (except for case-sensitive code), which seems to be the majority convention in the documentation.

• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

This PR enables one to build the documentation on Windows by manually invoking sphinx-build.

The first commit enables sphinx to execute the "conf.py" file. Before the commit, sphinx complains:

Configuration error: There is a programmable error in your configuration file:

with the exception FileNotFoundError: [WinError 2] The system cannot find the file specified where it tries to invoke conda. On Windows, conda environments other than the base environment have "conda.bat" on their path rather than "conda.exe", and one must call subprocess.run with "conda.bat" instead of merely "conda". However, the CONDA_EXE environment variable correctly points to the executable and should do so across platforms; it requires conda version 4.5 or later for conda/conda#6923.

The second commit enables the build to tolerate exceptions when deleting temporary files and tells Git to ignore more such files. Without the commit, builds occasionally failed at the os.remove calls with exceptions such as PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'example.nc'.

Should "whats-new.rst" mention these changes, which are developer-visible but not user-visible?

I welcome any feedback.

Edit: Checked the "whats-new" task.

Lengthening the underline resolves the following warning from sphinx-build:

``` [...]\xarray\doc\whats-new.rst:20: WARNING: Title underline too short.

v2022.03.1 (unreleased)

```

• [X] User visible changes (including notable bug fixes) are documented in whats-new.rst

Previously, on Windows, the subject test unnecessarily failed, and the temporary directory and file remained, because the scheduler in the outer context prevented deleting the temporary directory upon exiting the inner context of the latter.

The first commit swaps the two contexts to resolve the issue. The second commit replaces the tempfile.TemporaryDirectory() context manager with pytest's tmp_path fixture, which slightly simplifies the test and relies on pytest to remove the directory on a subsequent invocation.