issues

What happened:

open_dataset doesn't replace ~ any more

What you expected to happen:

should replace ~ for backward compatibility

Minimal Complete Verifiable Example:

```python import xarray as xr

da = xr.DataArray([1, 2, 3]) da.to_netcdf("~/bug_report.nc")

xr.open_dataarray("~/bug_report.nc") ```

Anything else we need to know?: works on 0.17.0, fails on master

Is your feature request related to a problem? Please describe. In #4309 we proposed to separate xarray - backend tasks, more or less in this way: - Backend returns a dataset - xarray manage chunks and cache.

With the changes in open_dataset to support also zarr (#4187 ), we introduced a slightly different behavior for zarr chunking with respect the other backends.

Behavior of all the backends except zar - if chunk == {} or 'auto': it uses dask and only one chunk per variable - if the user defines chunks for not all the dimensions, along these dimensions it uses only one chunk: ```python

ds = xr.open_dataset('test.nc', chunks={'x': 4}) print(ds['foo'].chunks) ((4, 4, 4, 4, 4), (4,)) *Zarr chunking behavior* is very similar, but it has a different default when the user doesn't choose the size of the chunk along some dimensions, i.e. - if chunk == {} or 'auto': it uses in both cases the on-disk chunks - if the user defines the chunks for not all the dimensions, along these dimensions it uses no disk chunck:python ds = xr.open_dataset('test.zarr', engine='zarr', chunks={'x': 4}) print(ds['foo'].encoding['chunks']) (5, 2) print(ds['foo'].chunks) ((4, 4, 4, 4, 4), (2, 2)) ```

Describe the solution you'd like

We could extend easily zarr behavior to all the backends (which, for now, don't use the field variable.encodings['chunks']): if no chunks are defined in encoding, we use as default the dimension size, otherwise, we use the encoded chunks. So for now we are not going to change any external behavior, but if needed the other backends can use this interface. I have some additional notes:

• The key value auto is redundant because it has the same behavior as {}, we could remove one of them.
• I would separate the concepts "on disk chunk" and "preferred chunking". We can use a different key in encodings or ask the backend to expose a function to compute the preferred chunking.

One last question: - In the new interface of open_dataset there is a new key, imported from open_zarr: overwrite_encoded_chunks. Is it really needed? Why do we support to overwrite of the encoded chunks at readi time? This operation can be easily done after or at write time.