`to_zarr` with `mode='a'` fails when `_FillValue` is present, caused by `open_zarr` with `mask_and_scale=False`.

What happened?

When opening a Zarr dataset with open_zarr and then writing to it using to_zarr, we get a ValueError when the _FillValue attribute of at least one data variable or coordinate is present. This happens for example when opening the dataset with mask_and_scale=False.

ValueError: failed to prevent overwriting existing key _FillValue in attrs. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually.

The error can be worked around by deleting the _FillValue attributes of all data variables and coordinates before calling to_zarr. However, if the Zarr metadata contains meaningful fill_value attributes beforehand, they will then be lost after a round-trip of open_zarr (with mask_and_scale=False) and to_zarr.

The same behavior, but without the cause being mask_and_scale=False, has been reported in the open issues #6069 and #6329 without any solutions.

What did you expect to happen?

I expect to be able to read from and then write to a Zarr storage without having to delete attributes in between or lose available fill_value metadata from the Zarr storage. Calling to_zarr with mode='a' should just write any DataArrays _FillValue attribute to the fill_value field in the Zarr metadata instead of failing with a ValueError.

Minimal Complete Verifiable Example

import xarray as xr

# Create a dataset and write to Zarr storage

ds = xr.Dataset(dict(A=xr.DataArray([1.0])))
# ds.A.attrs is empty here.

zarr_path = "/path/to/storage.zarr"
ds.to_zarr(zarr_path, mode='a')

# Read the dataset from Zarr again using `mask_and_scale=False`

ds = xr.open_zarr(zarr_path, mask_and_scale=False)
# ds.A.attrs is now {'_FillValue': nan}

# Write the dataset to Zarr again
ds.to_zarr(zarr_path, mode='a')

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

No response

Anything else we need to know?

No response

Environment

Start by running the minimal example and tracing the open_zarr and to_zarr paths that handle _FillValue when mask_and_scale=False. Done means mode='a' completes without a ValueError and preserves meaningful fill_value metadata through the Zarr round trip.