Description
What happened?
When writing a Dataset to netcdf, any DataArrays that are linked as bounds through another variables attrs['bounds'] entry, have their (specifically) 'units' attribute dropped inside the written netcdf file.
See example
What did you expect to happen?
Units attribute to be written to the netcdf file.
Minimal Complete Verifiable Example
import numpy as np
import xarray as xr
# Create a new Dataset
ds = xr.Dataset()
# Add the x variable, Specify 'x_bnds' as bounds, defined later.
ds['x'] = xr.DataArray(np.arange(10), dims='x', attrs={'units':'m', 'bounds':'x_bnds'})
# Bounds require an extra dimension equal to number of vertices.
ds['nv'] = xr.DataArray(np.r_[0, 1], dims='nv')
# Add the actual bounding values for variable x.
ds['x_bnds'] = xr.DataArray(np.squeeze(np.dstack([np.arange(10)-0.5, np.arange(10)+0.5])),
dims=['x', 'nv'],
attrs={'test':4, 'units':'m', })
print('Units is attached to the bounds in the dataset before writing', 'units' in ds['x_bnds'].attrs)
# Write to netcdf file
ds.to_netcdf('tmp.nc', format='netcdf4', engine='netcdf4')
# Open the dataset and check x_bnds attrs. units is dropped.
new = xr.open_dataset('tmp.nc')
print(new['x_bnds'].attrs)
# Confirm that units were never written to the file.
!h5dump -d /x_bnds tmp.ncMVCE 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.
- Recent environment — the issue occurs with the latest version of xarray and its dependencies.
Relevant log output
No response
Anything else we need to know?
No response
Environment
INSTALLED VERSIONS
commit: None
python: 3.11.5 (main, Sep 11 2023, 08:19:27) [Clang 14.0.6 ]
python-bits: 64
OS: Darwin
OS-release: 21.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.2
libnetcdf: 4.9.3-development
xarray: 2023.10.1
pandas: 2.1.1
numpy: 1.26.1
scipy: 1.11.3
netCDF4: 1.6.4
pydap: None
h5netcdf: None
h5py: 3.10.0
Nio: None
zarr: None
cftime: 1.6.3
nc_time_axis: None
PseudoNetCDF: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.8.0
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 68.0.0
pip: 23.3
conda: None
pytest: None
mypy: None
IPython: 8.16.1
sphinx: 7.2.6