fix(type-inference)!: follow nested struct field references by alexandrefimov · Pull Request #294 · substrait-io/substrait-python

alexandrefimov · 2026-10-08T09:19:10Z

Follow child reference segments for row, outer and lambda references, returning the selected struct field, list element or map value type as specified by Substrait v0.101.0.

Closes #293

BREAKING CHANGE: Nested struct references now infer the terminal field type instead of the outer struct. Invalid field paths are rejected.

coderabbitai · 2026-10-08T09:19:30Z

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src/substrait/type_inference.py
tests/test_type_inference.py

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src/substrait/type_inference.py
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Bug Fixes
- Nested field references now resolve through structs, list elements, and map values, including supported row, lambda, and outer references.
- Invalid field indices, incompatible container traversal, and map keys that do not match the map’s key type now raise errors.
- Filtering on null list or map values and selecting accessed values now produces plans with the correct inferred output type.
- Type inference preserves the input row and expression while resolving nested references.

Walkthrough

Direct-reference type inference now traverses nested struct fields, list elements, and map values. It validates container types, struct-field bounds, and map-key literal types. Tests cover nested selections, nullability, invalid paths, and collection-access output types.

Changes

Nested direct-reference type inference

Layer / File(s)	Summary
Traverse and validate reference segments `src/substrait/type_inference.py`, `tests/test_type_inference.py`, `tests/dataframe/test_frame.py`	`infer_expression_type` follows struct-field, list-element, and map-value segments. It validates map-key literals against map key types. Tests cover nested paths, nullability, invalid indices and containers, and inferred collection-access types.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: nielspardon

Merge Risk: ⚪ Minimal · up to 77b2a

Type inference now returns the terminal field type for nested references and rejects invalid paths. This is the stated behavior change. No actionable merge-blocking risk was identified in the supplied context.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 13.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 15 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses valid Conventional Commit syntax and clearly identifies the main change: nested struct field reference type inference. The breaking-change marker matches the stated behavior change.
Description check	✅ Passed	The description explains the implementation goal, identifies the affected reference types, states the breaking behavior, and includes the required BREAKING CHANGE footer and issue reference.
Linked Issues check	✅ Passed	Issue `#293` requires `infer_expression_type` to follow `ReferenceSegment.StructField.child` and return the terminal type for all field-reference roots. `src/substrait/type_inference.py` now traverses c…
Out of Scope Changes check	✅ Passed	The production change is limited to chained reference type inference. The tests directly verify the linked behavior and its invalid-input handling. List and map traversal support the same child-segmen…

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nielspardon

Thanks, this fixes #293 nicely. One regression to sort out before merge: a struct field whose child is a list_element or map_key now raises, which breaks col("xs")[0] and .map_key(...) in the DataFrame API — see inline.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/substrait/type_inference.py:
- Around line 665-669: In the map_key branch of infer_expression_type, validate
the inferred type of segment.map_key against result.map.key and reject
incompatible keys before assigning result.map.value. Preserve the existing
map-kind check and value-type selection for compatible keys.

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📒 Files selected for processing (3)

src/substrait/type_inference.py
tests/dataframe/test_frame.py
tests/test_type_inference.py

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nielspardon

Thanks — the list and map handling looks good. One side effect of the strict map-key check: col("m").map_key(7) on a map whose key isn't i64 now raises, because Expr.map_key always builds an i64 literal. That's a builder issue rather than a problem with this PR, so I've filed #295 for it.

alexandrefimov · 2026-10-08T17:00:13Z

Could you please rerun the cancelled macOS job?

nielspardon

LGTM

fix(type-inference)!: follow nested struct field references

ac8afa5

nielspardon reviewed Oct 8, 2026

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Comment thread src/substrait/type_inference.py Outdated

alexandrefimov added 2 commits October 8, 2026 16:49

fix(type-inference): resolve list and map reference children

19475ec

test(type-inference): use matching map key types

b154ec0

coderabbitai Bot reviewed Oct 8, 2026

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Comment thread src/substrait/type_inference.py

fix(type-inference): validate map key literal types

77b2a02

nielspardon mentioned this pull request Oct 8, 2026

Expr.map_key builds its key literal without regard to the map's key type #295

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nielspardon reviewed Oct 8, 2026

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nielspardon merged commit 3908dac into substrait-io:main Oct 8, 2026
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fix(type-inference)!: follow nested struct field references - #294

fix(type-inference)!: follow nested struct field references#294
nielspardon merged 4 commits into
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alexandrefimov:nested-field-type-inference

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alexandrefimov commented Oct 8, 2026 •

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