René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Bug: TypeError / ValueError when materializing Array(String) feature views with Athena offline store","articleBody":"## Description\n\nMaterializing feature views that contain `Array(String)` columns using the Athena offline store fails intermittently with one of two errors:\n\n### Error 1: `ValueError: The truth value of an empty array is ambiguous`\n\nTriggered when an entity row has no values set for an array column (e.g. `tags = []`).\n\n```\nFile \".../feast/type_map.py\", line 772, in _python_value_to_proto_value\n    elif not pd.isnull(value):\nValueError: The truth value of an empty array is ambiguous. Use `array.size \u003e 0` to check that an array is not empty.\n```\n\n### Error 2: `TypeError: bad argument type for built-in operation`\n\nTriggered when any user has values in an array column.\n\n```\nFile \".../feast/type_map.py\", line 905, in \u003clistcomp\u003e\n    ProtoValue(**{field_name: proto_type(val=value)})\nTypeError: bad argument type for built-in operation\n```\n\n## Root Cause\n\nArrow/Athena deserializes `Array(String)` feature columns as `numpy.ndarray` with `object` dtype rather than plain Python lists. Two code paths in `type_map.py` do not handle this:\n\n1. **Scalar null-check** (`_convert_scalar_values_to_proto`): The line `elif not pd.isnull(value)` calls `pd.isnull()` on a numpy array, which returns an array of bools — then `not \u003carray\u003e` raises `ValueError` because the truth value is ambiguous.\n\n2. **Generic list conversion** (`_convert_list_values_to_proto`): The call `proto_type(val=value)` passes the raw `numpy.ndarray` directly to the protobuf constructor. Protobuf rejects non-list types and raises `TypeError`. Additionally, Arrow nullable columns can produce `None` elements inside the ndarray, which protobuf `StringList` also rejects.\n\n3. **Type validation** (`_validate_collection_item_types`): `None` elements inside an ndarray fail the `type(item) in valid_types` check before they can be sanitized downstream.\n\n## Steps to Reproduce\n\n1. Define a `FeatureView` with an `Array(String)` field:\n   ```python\n   Field(name=\"tags\", dtype=Array(String))\n   ```\n2. Materialize from Athena where some rows have non-empty arrays, some have empty arrays, and some have NULL values in array elements.\n3. Observe `ValueError` or `TypeError` in `feast/type_map.py`.\n\n## Expected Behavior\n\nMaterialization completes successfully. Array columns from Arrow/Athena are converted to proto-safe Python lists, with `None` elements replaced by empty string.\n\n## Environment\n\n- Feast version: (any version with the generic list proto conversion)\n- Offline store: Athena\n- Python: 3.11\n- Feature column type: `Array(String)` (maps to `ValueType.STRING_LIST`)\n\n## Fix\n\nI opened a PR with a fix for this issue: https://github.com/feast-dev/feast/pull/6324","author":{"url":"https://github.com/alan-gauthier-jt","@type":"Person","name":"alan-gauthier-jt"},"datePublished":"2026-04-24T08:19:34.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/6325/feast/issues/6325"}