How zarr.py Handles Zarr V2 Stores

ZarrParser should now support both Zarr V2 and V3 stores by normalizing V2 stores to appear as V3. This approach ensures that all parsers produce V3-compatible outputs, and confines modifications to zarr.py.

V2 → V3 Normalization Strategy

The parser performs a two-part normalization:

1. Chunk Key Mapping (get_chunk_mapping_prefix)

For V2 arrays:

• Chunk files are stored directly under the array path: array_name/0, array_name/0.1.2
• Metadata files (.zarray, .zattrs, etc.) are filtered out
• Chunk coordinates are normalized to dot-separated format: "0.1.2"
• File paths in the manifest point to the actual V2 chunk locations
• Manifest keys contain only chunk coordinates (no path structure)

2. Metadata Conversion (get_metadata())

After converting V2 metadata to V3 using _convert_array_metadata, we have to replace the chunk_key_encoding.

• The automatic converter preserves V2ChunkKeyEncoding in the V3 metadata
• When zarr/xarray sees V2ChunkKeyEncoding, it requests chunks using V2-style paths: array/0
• With DefaultChunkKeyEncoding, zarr requests chunks using V3-style paths: array/c/0
• ManifestStore.get() expects V3-style paths and uses parse_manifest_index() to extract chunk coordinates
• parse_manifest_index() requires the /c/ component to correctly parse the path

Additional metadata handling

• None fill values: Converted to appropriate dtype defaults
• Dimension names: Extracted from _ARRAY_DIMENSIONS attribute or generated as {array_name}_dim_{i}
• All other metadata: Converted using zarr's standard V2→V3 migration utilities

Implementation Notes

I'm not convinced I've done a particularly elegant implementation here, but adding another class for V2 parsing didn't seem like it would be particularly extensible. Very happy to hear thoughts on perhaps a better implementation.

@TomNicholas thank you very much for your feedback up there, definitely helped me wrap my head around the right approach to take here.

Edit: I've done a bit of re-design to use a strategy pattern for dispatching to parsing v2 and v3 arrays. This should make future integrations of zarr array version parsing a lot more maintainable. This is also just a lot easier to read than my original implementation. Tests and such are also up to date. Also going to move this into the PR notes instead of huge comment here.

Let me know when you would like a review of this @neilSchroeder !

@TomNicholas I think it's ready for a review.

Codecov Report

❌ Patch coverage is 99.13793% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.31%. Comparing base (cb2912e) to head (a4a271f).
⚠️ Report is 1 commits behind head on main.

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+ Hits         1649     1738      +89     
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@TomNicholas I'm ready for another review whenever you've got time

@TomNicholas what are the next steps here? Will this be merged whenever someone has time to do the next release? Do we need another reviewer?