Thanks @kmuehlbauer .

@yt87 please reopen if you still have trouble after upgrading h5netcdf

@yt87 This works for me with latest h5netcdf version. There have been several fixes regarding attribute reading. The issue might be closed after confirming.

Possibly related issue: https://github.com/h5netcdf/h5netcdf/issues/27

Anyway, I think I've found the root-cause of the issue. So with netcdf4-python it is possible to specify writing of attributes as NC_STRING string attributes.

The following will create a netcdf file which is readable with current xarray and h5netcdf backend.

```python import netCDF4 as nc import xarray as xr rootgrp = nc.Dataset("test_nc4_attrs.nc", mode="w", format="NETCDF4")

rootgrp.set_ncstring_attrs(True)

x = rootgrp.createDimension("x", 1) y = rootgrp.createDimension("y", 1) foo = rootgrp.createVariable("foo", "i4", ("y", "x")) foo.coordinates = "y x" foo[0, 0] = 0 rootgrp.close() ds = xr.open_dataset("test_nc4_attrs.nc", engine="h5netcdf") print(ds) print(ds.foo) ds.close() ```

If you uncomment the one line, this creates netcdf file which breaks with the error @yt87 showed above. If you try to open the file with decode_cf=False and write it back using to_netcdf the same error occurs.

h5dump before uncomment ATTRIBUTE "coordinates" { DATATYPE H5T_STRING { STRSIZE 3; STRPAD H5T_STR_NULLTERM; CSET H5T_CSET_ASCII; CTYPE H5T_C_S1; } DATASPACE SCALAR DATA { (0): "y x" } }

h5dump after uncomment ATTRIBUTE "coordinates" { DATATYPE H5T_STRING { STRSIZE H5T_VARIABLE; STRPAD H5T_STR_NULLTERM; CSET H5T_CSET_UTF8; CTYPE H5T_C_S1; } DATASPACE SIMPLE { ( 1 ) / ( 1 ) } DATA { (0): "y x" } }

Another thing I found, that this only accounts for coordinate attributes. All other attributes are read correctly AFAIKT. So with this in mind I would consider that a bug. The assumption that attributes are always scalar doesn't hold true.

And one more thing, also roundtripping in netcdf4 does not keep the NC_STRING type:

```python import netCDF4 as nc import xarray as xr rootgrp = nc.Dataset("test_nc4_attrs.nc", mode="w", format="NETCDF4") rootgrp.set_ncstring_attrs(True) x = rootgrp.createDimension("x", 1) y = rootgrp.createDimension("y", 1) foo = rootgrp.createVariable("foo", "i4", ("y", "x")) rootgrp.coordinates = "z y x" foo.coordinates = "y x" foo[0, 0] = 0 rootgrp.close()

ds = xr.open_dataset("test_nc4_attrs.nc", engine="netcdf4") ds.to_netcdf("test_nc4_attrs_out.nc") ds.close() ```

``` ncdump test_nc4_attrs.nc

netcdf test_nc4_attrs { dimensions: x = 1 ; y = 1 ; variables: int foo(y, x) ; string foo:coordinates = "y x" ;

// global attributes: string :coordinates = "z y x" ; data:

foo = 0 ; } ```

``` ncdump test_nc4_attrs_out.nc

netcdf test_nc4_attrs_out { dimensions: y = 1 ; x = 1 ; variables: int foo(y, x) ; foo:coordinates = "y x" ; data:

foo = 0 ; } ```

So changes are needed in several places to check if the attribute is scalar or array, not only for coordinates attributes. Thoughts?

You might be right. Adding -k nc4 works when string is removed from attribute specification. If it is present, the error is as before: AttributeError: 'numpy.ndarray' object has no attribute 'split'.

However, after changing my AWS script to ``` import s3fs import xarray as xr

s3 = s3fs.S3FileSystem(anon=True) s3path = 's3://wrf-se-ak-ar5/gfdl/hist/daily/1988/WRFDS_1988-04-23.nc'

ds = xr.open_dataset(s3.open(s3path), engine='scipy') print(ds) ` the error isTypeError: Error: None is not a valid NetCDF 3 file``.

@yt87 Thanks for providing all this.

Could you please give -k 'nc4' as option to the call to ncgen and report back? I think this is a problem with data model.

This is how I did it:

``` $ ncdump /tmp/x.nc netcdf x { dimensions: x = 1 ; y = 1 ; variables: int foo(y, x) ; foo:coordinates = "x y" ; data:

foo = 0 ; } $ rm x.nc $ ncgen -o x.nc < x.cdl $ python -c "import xarray as xr; ds = xr.open_dataset('/tmp/x.nc', engine='h5netcdf'); print(ds)" ``` Engine netcdf4 works fine, with string or without.

My original code retrieving data from AWS: ``` import s3fs import xarray as xr

s3 = s3fs.S3FileSystem(anon=True) s3path = 's3://wrf-se-ak-ar5/gfdl/hist/daily/1988/WRFDS_1988-04-23.nc'

ds = xr.open_dataset(s3.open(s3path)) print(ds) ` Addingdecode_cf=False`` is a workaround. All attributes are arrays:

Attributes: contact: ['rtladerjr@alaska.edu'] info: ['Alaska CASC'] data: ['Downscaled GFDL-CM3'] format: ['version 2'] date: ['Mon Jul 1 15:17:16 AKDT 2019']

If you want a quick fix to read the file, add kwarg decode_cf=False in open_dataset. Then fix the coordinate attribute and call xr.decode_cf(ds).

Hmm, this worked in my case. How exactly did you create the netcdf file?

Dropping string changes error to Unable to open file (file signature not found). This issue popped up while reading data from https://registry.opendata.aws/wrf-se-alaska-snap/

@yt87 AFAIK the CDL doesn't declare attribute types specifically. This should work:

``` netcdf x { dimensions: x = 1 ; y = 1 ; variables: int foo(y, x) ; foo:coordinates = "x y" ; data:

foo = 0 ; } ```