feat(capi): implement PySequence_Fast for fast sequence extraction by krosci · Pull Request #8937 · RustPython/RustPython

krosci · 2026-10-01T17:01:13Z

This change implements the PySequence_Fast C-API function to quickly return lists and tuples directly or extract iterable elements into a new list with custom error messages on failure, addressing part of #8156.

Summary by CodeRabbit

New Features
- Added PySequence_Fast to convert iterable objects into lists while returning list and tuple inputs unchanged.
- When an object cannot be iterated, callers can now provide a custom TypeError message. Errors raised while iterating are preserved.

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crates/capi/src/abstract_/sequence.rs

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crates/capi/src/abstract_/sequence.rs

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📝 Walkthrough

Walkthrough

Adds the C API function PySequence_Fast. It returns list and tuple inputs by reference, converts other iterable inputs into a list, and supports a custom error message when iterator acquisition fails with TypeError.

Changes

PySequence_Fast

Layer / File(s)	Summary
Function behavior and tests `crates/capi/src/abstract_/sequence.rs`	Adds `PySequence_Fast` with list and tuple identity handling, conversion of other iterable inputs, and error handling. Tests cover identity, conversion, custom error text, and propagation of errors raised during iteration.

Priority: ⬇️ Low

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

Change: Feature

Merge Risk: ⚪ Minimal · up to 8983d

PySequence_Fast implements the expected conversion and error behavior. No actionable merge-blocking issue was found; merge after normal checks pass.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 06fc3

The new API stays within the existing interpreter boundary and preserves owned-reference handling. However, supplying a custom error message also replaces iterator failures and interruption exceptions, potentially weakening callers’ failure handling. No privilege escalation or affected production consumer was established.

Retained concerns

Medium · reliability · inferred: With a custom message, the new entrypoint replaces exceptions from iterator acquisition and iteration with TypeError. This erases interruption and failure identity at the native API boundary, potentially allowing callers that recover from invalid input to mishandle failures that should terminate processing. No affected production caller or security exploit was established.

Security review details

Security Blast Radius

inferred — The demonstrated exposure is an in-process native API capable of executing supplied iterable behavior in the existing VM. Supplied caller evidence demonstrates tests, not a production consumer; external extensions and embedding environments remain outside coverage.

Security Findings and Attack Paths

inferred — A supplied iterator can raise an exception during iteration that the wrapper relabels as TypeError when a custom message is present. A caller that treats TypeError as recoverable validation failure could then mishandle interruption or another terminal failure. This is a potential containment path, not a verified exploit or authority bypass.

Trust Boundaries and Controls

observed — Iterable execution follows existing VM object and iterator protocols rather than a separate execution mechanism. The new native-facing wrapper subsequently determines which exception crosses back to its caller.

Resilience and Maintainability Implications

observed — The null-message branch preserves the original exception, and the common FFI result adapter publishes that exception with a null pointer. Custom-message substitution changes exception identity but does not turn the failed conversion into a successful result.

Hardening Proposals

proposed — Separate initial non-iterability from iteration-time failure, limiting custom-message substitution to the intended initial TypeError while preserving interruption and other iterator exceptions.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: implementing `PySequence_Fast` in the C API for fast sequence extraction.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
Review comments at @crates/capi/src/abstract_/sequence.rs:
- Line 195: Separate iterator acquisition from iterable consumption in the
sequence conversion path guarded by `if !m.is_null()`: apply the custom message
only to a `TypeError` raised while acquiring the iterator, and propagate all
errors raised while consuming it unchanged. Add regression tests for both
acquisition and consumption errors.

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crates/capi/src/abstract_/sequence.rs

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codspeed · 2026-10-01T18:29:45Z

Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 4 skipped benchmarks¹

_{Comparing krosci:feat/capi-abstract-sequence-fast (8983d32) with main (112b7ef)}

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

Assisted-by: Gemini:gemini-3.7-flash

coderabbitai Bot reviewed Oct 1, 2026

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Comment thread crates/capi/src/abstract_/sequence.rs Outdated

feat(capi): implement PySequence_Fast for fast sequence extraction

18a06db

Assisted-by: Gemini:gemini-3.7-flash

krosci force-pushed the feat/capi-abstract-sequence-fast branch from 8983d32 to 18a06db Compare October 1, 2026 18:30

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feat(capi): implement PySequence_Fast for fast sequence extraction - #8937

feat(capi): implement PySequence_Fast for fast sequence extraction#8937
krosci wants to merge 1 commit into
RustPython:mainfrom
krosci:feat/capi-abstract-sequence-fast

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codspeed Bot commented Oct 1, 2026

Merging this PR will not alter performance

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krosci commented Oct 1, 2026 •

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