issue_comments: 1081139207

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https://github.com/pydata/xarray/issues/6374#issuecomment-1081139207 https://api.github.com/repos/pydata/xarray/issues/6374 1081139207 IC_kwDOAMm_X85AcOAH 22245117 2022-03-28T21:01:19Z 2022-03-28T21:01:19Z CONTRIBUTOR

Adding support for reading NCZarr in the "zarr" backend should be quite easy if xarray doesn't need to integrate the additional features in NCZarr (e.g., groups, fully qualified names, dtypes for attributes). It looks like the main difference is that the dimension names stored by xarray in .zattrs["_ARRAY_DIMENSIONS"] are stored by NCZarr in .zarray["_NCZARR_ARRAY"]["dimrefs"]. I drafted PR #6420 to explore what it would take to support reading NCZarr in xarray's "zarr" backend, and I don't think there are major changes/additions needed. (I'm experiencing issues with Windows in PR #6420. I think they need to be explored in netcdf4-python or netcdf-c though - I've added a comment in the PR)

I'm not sure whether it is better to (i) add direct support for NCZarr in xarray or (ii) just rely on the netcdf4 backend. After playing a bit with both backends, I have a few comments if option (ii) is chosen: * I would change the error raised when "_ARRAY_DIMENSIONS" is not present, suggesting to try the netcdf4 backend as well. Also, I think it's worth pointing out in the documentation or in the error message where to find information on how to open/write zarr data with the netcdf4 backend. I suspect right now it's not easy to find that information for python/xarray users. * I would consider starting a deprecation cycle for open_zarr, so it will be more clear that zarr data can be opened using various backends. * If "_ARRAY_DIMENSIONS" and "_NC*" attributes will coexist in the next version of NCZarr, the zarr backend will be able to open NCZarr but will treat "_NC*" attributes as regular attributes. I think the "zarr" backend would have to handle "_NC*" attributes (e.g., drop or hide), otherwise there can be issues when writing: TypeError: Invalid value for attr '_NCZARR_ATTR': {'types': {'Conventions': '<U1', 'title': '<U1', 'description': '<U1', 'platform': '<U1', 'references': '<U1', '_NCProperties': '<U1'}}. For serialization to netCDF files, its value must be of one of the following types: str, Number, ndarray, number, list, tuple

 
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