What happened?

As per: https://github.com/pydata/xarray/pull/8246#discussion_r1340357405

Passing a negative step in a slice to select a non-chunked zarr-backed datasets raises an error.

What did you expect to happen?

zarr should allow negative step (probably?)

Minimal Complete Verifiable Example

```Python import xarray as xr

create a zarr dataset

air = xr.tutorial.open_dataset("air_temperature") air.to_zarr("test.zarr")

ds = xr.open_dataset("test.zarr", engine="zarr") ds.air[::-1, ].load()

note that this works if the dataset is backed by dask

ds_dask = xr.open_dataset("test.zarr", engine="zarr", chunks="auto") ds_dask.air[::-1, ].load() ```

MVCE confirmation

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

Relevant log output

```Python File ~/code/xarray/xarray/core/parallelcompat.py:93, in guess_chunkmanager(manager) 91 if isinstance(manager, str): 92 if manager not in chunkmanagers: ---> 93 raise ValueError( 94 f"unrecognized chunk manager {manager} - must be one of: {list(chunkmanagers)}" 95 ) 97 return chunkmanagers[manager] 98 elif isinstance(manager, ChunkManagerEntrypoint): 99 # already a valid ChunkManager so just pass through

ValueError: unrecognized chunk manager dask - must be one of: [] ```

Anything else we need to know?

The error comes from https://github.com/zarr-developers/zarr-python/blob/6ec746ef1242dd9fec26b128cc0b3455d28ad6f0/zarr/indexing.py#L174 so it would need an upstream fix first.

cc @dcherian is this what you had in mind?

Environment

INSTALLED VERSIONS ------------------ commit: f6d69a1f6d952dcd67609c97f3fb3069abdda586 python: 3.10.12 | packaged by conda-forge | (main, Jun 23 2023, 22:40:32) [GCC 12.3.0] python-bits: 64 OS: Linux OS-release: 6.2.0-33-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.14.2 libnetcdf: 4.9.2 xarray: 2023.9.1.dev8+gf6d69a1f pandas: 2.1.1 numpy: 1.24.4 scipy: 1.11.3 netCDF4: 1.6.4 pydap: installed h5netcdf: 1.2.0 h5py: 3.9.0 Nio: None zarr: 2.16.1 cftime: 1.6.2 nc_time_axis: 1.4.1 PseudoNetCDF: 3.2.2 iris: 3.7.0 bottleneck: 1.3.7 dask: 2023.9.2 distributed: None matplotlib: 3.8.0 cartopy: 0.22.0 seaborn: 0.12.2 numbagg: 0.2.2 fsspec: 2023.9.2 cupy: None pint: 0.20.1 sparse: 0.14.0 flox: 0.7.2 numpy_groupies: 0.10.1 setuptools: 68.2.2 pip: 23.2.1 conda: None pytest: 7.4.2 mypy: None IPython: 8.15.0 sphinx: None