Preserve KeyError for missing artifacts

[talsperre]

Summary

• remove the DataException wrapper for artifacts missing from _objects
• let the natural _objects[name] lookup preserve the historical KeyError behavior
• keep the old metadata fallback path: if an artifact has an object entry but no _info entry, load it with very old pickle defaults
• add focused serializer integration coverage for missing artifact, missing metadata, and metadata-without-object cases

Context

The pluggable serializer load path already uses self._info.get(name, {}), which is the right compatibility behavior for artifacts without metadata: assume the very old pickle serializer defaults. The regression came from wrapping the later _objects[name] miss in a DataException, which made truly absent artifacts look like metadata/blob corruption.

Validation

• python -m pytest test/unit/test_serializer_integration.py::test_missing_artifact_raises_key_error test/unit/test_serializer_integration.py::test_missing_info_with_object_uses_pickle_defaults test/unit/test_serializer_integration.py::test_info_without_object_raises_key_error -q
• python -m pytest test/unit/test_serializer_integration.py -q
• pre-commit run --files metaflow/datastore/task_datastore.py test/unit/test_serializer_integration.py

[greptile-apps]

Greptile Summary

This PR fixes a regression in the pluggable-serializer load path where self._info.get(name, {}) caused truly-absent artifacts to masquerade as having default metadata, turning their _objects miss into a DataException (corruption report) instead of the expected KeyError. The fix removes the surrounding try/except and lets the raw KeyError from self._objects[name] propagate for both the absent-artifact and the metadata-with-no-blob cases.

• task_datastore.py: Drops the try/except KeyError → DataException wrapper around to_load[self._objects[name]].append(name), so both the "artifact was never saved" and "metadata exists but no stored blob" paths now raise a bare KeyError.
• test_serializer_integration.py: Adds three focused tests: missing artifact raises KeyError, missing _info entry falls back to pickle defaults, and metadata-only artifact raises KeyError.

Confidence Score: 5/5

Safe to merge; the one-line change is narrowly scoped, the tests directly cover both affected paths, and no existing callers of load_artifacts catch DataException specifically for the corruption case.

The change is minimal and well-tested. Both the absent-artifact and metadata-without-blob paths now consistently raise KeyError, matching the historical contract. The only outstanding item is a documentation discrepancy in the PR description which does not affect runtime behavior.

No files require special attention.

Important Files Changed

Filename	Overview
metaflow/datastore/task_datastore.py	Removes try/except around self._objects[name] so a bare KeyError propagates for both truly-absent artifacts and the metadata-with-no-blob corruption case, restoring the historical contract.
test/unit/test_serializer_integration.py	Adds three new tests covering the missing-artifact KeyError path, the no-metadata-with-object pickle-default path, and the metadata-only KeyError path; tests accurately reflect the new code behavior.

_{Reviews (2): Last reviewed commit: "Preserve KeyError for missing artifact b..." | Re-trigger Greptile}