SingleHdf5ToZarr error/warning with scalar utf-8/ascii dataset

When I try to run SingleHdf5ToZarr on an HDF5 file with a scalar HDF5 dataset that has a variable length utf-8 string dtype or a variable length ascii bytes dtype, I get the following warning that an error was caught:

/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/kerchunk/hdf.py:497: UserWarning: The following excepion was caught and quashed while traversing HDF5
'str' object has no attribute 'extend'
Traceback (most recent call last):
  File "/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/kerchunk/hdf.py", line 438, in _translator
    za = self._zroot.create_dataset(
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/hierarchy.py", line 1094, in create_dataset
    return self._write_op(self._create_dataset_nosync, name, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/hierarchy.py", line 935, in _write_op
    return f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^
  File "/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/hierarchy.py", line 1110, in _create_dataset_nosync
    a = array(data, store=self._store, path=path, chunk_store=self._chunk_store, **kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/rly/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/creation.py", line 439, in array
    z[...] = data
    ~^^^^^
AttributeError: 'str' object has no attribute 'extend'

  warnings.warn(msg)

When I pass error="raise" to SingleHdf5ToZarr, I see the source of the error in numcodecs/json.py:

...
File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/creation.py:439, in array(data, **kwargs)
    436 z = create(**kwargs)
    438 # fill with data
--> 439 z[...] = data
    441 # set read_only property afterwards
    442 z.read_only = read_only

File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/core.py:1497, in Array.__setitem__(self, selection, value)
   1495     self.set_orthogonal_selection(pure_selection, value, fields=fields)
   1496 else:
-> 1497     self.set_basic_selection(pure_selection, value, fields=fields)

File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/core.py:1591, in Array.set_basic_selection(self, selection, value, fields)
   1589 # handle zero-dimensional arrays
   1590 if self._shape == ():
-> 1591     return self._set_basic_selection_zd(selection, value, fields=fields)
   1592 else:
   1593     return self._set_basic_selection_nd(selection, value, fields=fields)

File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/core.py:1974, in Array._set_basic_selection_zd(self, selection, value, fields)
   1971         pass
   1972 else:
   1973     # encode and store
-> 1974     cdata = self._encode_chunk(chunk)
   1975     self.chunk_store[ckey] = cdata

File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/zarr/core.py:2436, in Array._encode_chunk(self, chunk)
   2434 if self._filters:
   2435     for f in self._filters:
-> 2436         chunk = f.encode(chunk)
   2438 # check object encoding
   2439 if ensure_ndarray_like(chunk).dtype == object:

File ~/mambaforge/envs/kerchunk/lib/python3.11/site-packages/numcodecs/json.py:59, in JSON.encode(self, buf)
     57 buf = np.asarray(buf)
     58 items = buf.tolist()
---> 59 items.extend((buf.dtype.str, buf.shape))
     60 return self._encoder.encode(items).encode(self._text_encoding)

AttributeError: 'str' object has no attribute 'extend'

Here, buf starts as a string. items is a string. It seems like JSON.encode assumes buf is array-like.

Example code to generate the HDF5 files:

import h5py

H5_STR = h5py.string_dtype("utf-8")
H5_BYTES = h5py.string_dtype("ascii")

with h5py.File("test_str.h5", "w") as f:
    f.create_dataset("data", data="test", shape=None, dtype=H5_STR)

with h5py.File("test_bytes.h5", "w") as f:
    f.create_dataset("data", data=b"test", shape=None, dtype=H5_BYTES)

Example code to generate the kerchunk reference JSON:

from kerchunk.hdf import SingleHdf5ToZarr
import fsspec
import ujson

fs_read = fsspec.filesystem('')  # local file system to read from
fs_write = fsspec.filesystem('')  # local file system to save final jsons to

def gen_json_from_local(local_file_path, final_remote_url, outf):
    with fs_read.open(local_file_path, 'rb') as infile:
        h5chunks = SingleHdf5ToZarr(infile, final_remote_url, inline_threshold=300, error="raise")

        with fs_write.open(outf, 'wb') as f:
            f.write(ujson.dumps(h5chunks.translate()).encode())

local_file_path = "/Users/rly/Documents/NWB/kerchunk-playground/test_str.h5"
final_remote_url = "s3://..."
outf = "test_str.json"  # file name to save json to
gen_json_from_local(local_file_path, final_remote_url, outf)

local_file_path = "/Users/rly/Documents/NWB/kerchunk-playground/test_bytes.h5"
final_remote_url = "s3://..."
outf = "test_bytes.json"  # file name to save json to
gen_json_from_local(local_file_path, final_remote_url, outf)

Start in kerchunk/hdf.py at _translator, then reproduce the issue with the provided h5py scalar UTF-8 and ASCII dataset examples using SingleHdf5ToZarr with error="raise". Verify that translating both files completes without the warning or exception and that the resulting references preserve the scalar string values.