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rustpython-capi-expansion

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Implement missing RustPython C-API functions in crates/capi using the pyo3-ffi header split mapping (`pyo3-ffi/src/*.rs`, mirroring CPython C API headers). Use this whenever the user asks to add or port C-API functions (for example from setobject.h, dictobject.h, unicodeobject.h) or add capi tests.

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Source SKILL.md: https://github.com/RustPython/RustPython/blob/HEAD/.agents/skills/rustpython-capi-expansion/SKILL.md

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RustPython C-API Expansion

Use this workflow for adding missing C-API functions to RustPython.

Source of truth for target files

  • Use this mapping source: pyo3-ffi/src/*.rs, which mirrors the CPython header split used by the C API.
  • Map requested header APIs to crates/capi/src/<header_basename>.rs using that split. Examples:
    • setobject.h -> crates/capi/src/setobject.rs
    • dictobject.h -> crates/capi/src/dictobject.rs
    • unicodeobject.h -> crates/capi/src/unicodeobject.rs
  • Do not invent alternate target modules when the header split implies a direct target.
  • If the target file is not present yet, create it and wire it in crates/capi/src/lib.rs.

Implementation workflow

  1. Identify requested missing APIs from the user request and their originating C API header.
  2. Open nearby capi modules in crates/capi/src/ and follow existing style and patterns.
  3. Implement only the requested functions in the mapped target file.
  4. Keep behavior aligned with CPython C-API contracts.
  5. Prefer using existing rustpython-vm functionality as much as possible instead of re-implementing behavior in capi.
  6. If a needed rustpython-vm helper exists but is private, make it public with a minimal, focused visibility change.
  7. Prefer direct contract assumptions over defensive null checks unless required by the established local style.
  8. Add basic tests only; do not overfit with very specific edge-case clutter.
  9. In tests, use pyo3 only as a safe wrapper over the API. Avoid raw pointer-heavy direct FFI-style tests.
  10. Run tests from crates/capi.

Testing rules

  • Run test commands with working directory set to crates/capi.
  • Prefer targeted tests first (module/function filter), then broader capi tests if needed.
  • Keep test names concise (no required test_ prefix).

Style rules

  • Follow existing RustPython capi coding style in neighboring files.
  • Reuse rustpython-vm methods and types first; avoid duplicating VM logic in capi wrappers.
  • When exposing previously private VM helpers, keep the API surface minimal and avoid unrelated refactors.
  • Only expose and implement ABI-stable C-API surface needed for abi3 / abi3t.
  • Add comments only when they explain non-obvious behavior.
  • Keep edits minimal and focused on requested API expansion.

Completion checklist

  • All requested functions implemented in mapped target file.
  • New module exported in crates/capi/src/lib.rs when applicable.
  • Basic safe-wrapper pyo3 tests added/updated.
  • Tests executed from crates/capi and passing for changed area.
  • Final response includes changed file paths and test command summary.

Example prompts this skill should handle

  • "Implement these missing functions from dictobject.h."
  • "Add setobject.h C-API functions in RustPython and include basic tests."
  • "Port the listed unicodeobject.h APIs in capi, follow existing style, and run tests from crates/capi."