Description
What happened?
I'm trying to convert a Zarr dataset in the Zarr format/version 3 to 2. Xarray can successfully read zarr v2 or v3. I can make an xarray dataset in memory and write to disc in either zarr v2 or v3. I seem to find this problem only when reading an existing zarr v3 and and attempting to write to a zarr v2. However i can successfully convert the opposite direction by reading an existing zarr v2 and writing to zarr v3. Given that all the other read/write scenarios that i've tried work fine, i'm guessing there's some internal config set when reading a dataset from zarr v3 which maybe doesn't exist or has an incompatible default for in zarr v2.
What did you expect to happen?
I expected this to be possible
xr.open_zarr('v3path.zarr', zarr_format=3).to_zarr('v2path.zarr', zarr_format=2)
Minimal Complete Verifiable Example
import xarray as xr
import numpy as np
# generate a random array and write it to zarr v3
arr = np.random.rand(100, 100)
(
xr
.DataArray(arr, dims=['x', 'y'], name='data')
.to_zarr(
'./test-v3.zarr',
zarr_format=3
)
)
# now try to convert it to a zarr 2
(
xr
.open_zarr('./test-v3.zarr')
.to_zarr('test-v2.zarr', zarr_format=2)
)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.
- Recent environment — the issue occurs with the latest version of xarray and its dependencies.
Relevant log output
ValueError: Zarr format 2 arrays do not support `serializer`.Anything else we need to know?
No response
Environment
This is on a google colab machine but i've also replicated this on so Mac and Windows installations
xarray: 2025.3.1
pandas: 2.2.2
numpy: 2.0.2
scipy: 1.15.3
netCDF4: None
pydap: None
h5netcdf: 1.6.1
h5py: 3.13.0
zarr: None
cftime: None
nc_time_axis: None
iris: None
bottleneck: 1.4.2
dask: 2024.12.1
distributed: 2024.12.1
matplotlib: 3.10.0
cartopy: None
seaborn: 0.13.2
numbagg: None
fsspec: 2025.3.2
cupy: 13.3.0
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 75.2.0
pip: 24.1.2
conda: None
pytest: 8.3.5
mypy: None
IPython: 7.34.0
sphinx: 8.2.3