feat(zarr): store the source array metadata document on read (provenance) - #2
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When a variable is read from a Zarr store, keep the array's complete metadata document on encoding["zarr_array_metadata"] for provenance and introspection. The key is read-only: it is listed in ZARR_READ_ONLY_ENCODING_KEYS, dropped on write, and never forwarded to zarr's array-creation call. The document is emitted by private per-format helpers (_v2_array_metadata, _v3_array_metadata) that read the metadata keys individually and return locally-defined TypedDicts mirroring the key names of the zarr-metadata package's ZarrV2ArrayMetadataJSON / ZarrV3ArrayMetadataJSON, with deliberately wide value types. These helpers are the only places xarray touches zarr-python's metadata API, so future changes to zarr-python's array class shape are absorbed there and surface loudly. Assisted-by: ClaudeCode:claude-fable-5 Co-authored-by: Claude <noreply@anthropic.com>
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Summary
When a variable is read from a Zarr store (under zarr-python 3), xarray now stores the array's complete metadata document —
zarr.Array.metadata.to_dict()— onencoding["zarr_array_metadata"], for provenance and introspection.It is a read-only encoding key: added to
ZARR_READ_ONLY_ENCODING_KEYS, so it is dropped on write and never forwarded to zarr's array-creation call (same treatment aspreferred_chunks/source). The write path is unchanged.This is deliberately a small, gradual first increment.
Why this scope
An earlier version of this branch made the fragment authoritative on write (driving array creation, converting v2⇄v3, folding codecs, stamping dtype/fill_value). Review — including several rounds of automated review — showed that faithful cross-format metadata conversion is a hard, spec-level problem with a large correctness surface, and it really belongs upstream in zarr-python rather than reimplemented in xarray. So we walked back to just storing the document without consuming it. v2⇄v3 conversion is explicitly out of scope here.
Follow-ups
zarr/metadata/migrate_v3.py::_convert_array_metadatato a public, bidirectional (v3→v2) converter with an explicit "raise when unconvertible" contract.zarr_array_metadataon write (built on that upstream converter) to enable v2⇄v3 round-trips.Testing
test_zarr_array_metadata_stored_on_read: opening a v3 store exposes the self-describing document on.encoding; rewriting succeeds (the read-only key is dropped, not passed tocreate) and reproduces the data. Fullpytest -k zarrgreen;mypyclean onzarr.py.Stacked on
claude/zarr-encoding-refactor.[This is Claude Code on behalf of Davis Vann Bennett]
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