Its possible to write zarr 2 format zarr's using zarr3 that can't be read by zarr v2

[benritchie]

Zarr version

v3.0.1

Numcodecs version

15.0

Python Version

3.12.8

Operating System

MacOs

Installation

conda

Description

Hi

I'm Loving Zarr v3 :) - thanks.

I'm trying ot use zarr 3 to create zarr 2 format Zarr's, via xarrray.

i'm using a numcodecs compressor (PCodec), and left the code in "zarr 3 style".

using this setup, zarr allows me to write the array in zarr 2 format, but when I try to read it, I get an error:

ImportError: zarr 3.0.0 or later is required to use the numcodecs zarr integration

I think zarr 3 should have some firewalling in to ensure that it only allows zarr 2 compatable compressors if writing a zarr2 array.

Steps to reproduce

zarr creation code below (run using zarr 3.0.1:

import zarr
import xarray
import numcodecs.zarr3 as numcodecsz3

#Open the zarr (any zarr should work here - just set "test_band_ below to a band from the zarr you are using)
ds = xarray.open_zarr('./hls_20200101_20201231_99cc_50q', consolidated=True)

# We want to write the DataSet with different encodings than the source, therefore
# clear all encoding config first.  
ds = ds.drop_encoding()

encoding = {}
encoding['test_band'] = {"serializer": numcodecsz3.PCodec(level=5)}

ds.to_zarr('./test2', mode="w", encoding = encoding, zarr_format=2)

read code:
(using latest version of zarr 2)

import zarr
import xarray
import numcodecs.zarr3 as numcodecsz3

# Open file
z2 = zarr.open('test2', mode='r')

print(z2['ndvi'][:])

Additional output

No response