sexp

name	sexp
description	How the Zig S-expression engine and typed KiCad models work, how they are exposed to Python (pyzig_sexp), and the invariants around parsing, formatting, and freeing. Use when working with KiCad file parsing, S-expression generation, or layout sync. Use when this capability is needed.
metadata	{"author":"atopile"}

Sexp Module

The sexp subsystem provides:

• a fast S-expression tokenizer/parser/pretty-printer in Zig, and
• typed Zig models for KiCad formats (PCB, footprint, netlist, symbol, schematic, fp_lib_table), exposed to Python via the pyzig_sexp extension module.

Source-of-truth docs and code:

• src/faebryk/core/zig/README.md (high-level overview)
• src/faebryk/core/zig/src/sexp/* (tokenizer/AST/structure)
• src/faebryk/core/zig/src/python/sexp/sexp_py.zig (Python API + critical memory rules)

Quick Start

from pathlib import Path
from faebryk.libs.kicad.fileformats import kicad

pcb = kicad.loads(kicad.pcb.PcbFile, Path("board.kicad_pcb"))
_text = kicad.dumps(pcb)

Relevant Files

• Zig core:
  • src/faebryk/core/zig/src/sexp/tokenizer.zig (tokenization + line/column tracking)
  • src/faebryk/core/zig/src/sexp/ast.zig (SExp tree + KiCad pretty formatting)
  • src/faebryk/core/zig/src/sexp/structure.zig (decode/encode + error context)
  • src/faebryk/core/zig/src/sexp/kicad/* (typed KiCad models)
• Python extension entrypoint:
  • src/faebryk/core/zig/src/python/sexp/init.zig (exports PyInit_pyzig_sexp)
  • src/faebryk/core/zig/src/python/sexp/sexp_py.zig (module + type binding generation)
• Generated Python stubs (what users “see”):
  • src/faebryk/core/zig/gen/sexp/*.pyi
• Convenience wrapper used throughout the codebase:
  • src/faebryk/libs/kicad/fileformats.py (namespaces modules + caching + loads/dumps)

Dependants (Call Sites)

• src/faebryk/libs/kicad/fileformats.py (primary integration layer)
• KiCad exporters and layout sync:
  • src/faebryk/exporters/pcb/kicad/*
  • src/faebryk/exporters/pcb/layout/layout_sync.py
• KiCad plugin workflow: src/atopile/kicad_plugin/*

How to Work With / Develop / Test

Core Concepts

• Two-level model:
  • raw SExp parsing/formatting (tokenizer.zig, ast.zig)
  • typed KiCad decoding/encoding (structure.zig + sexp/kicad/*.zig)
• Python API shape: the extension exposes per-format modules (e.g. pcb, netlist) with:
  • module-level loads(data: str) -> File
  • module-level dumps(file: File) -> str
  • File.free(...) for releasing Zig-owned allocations
• Convenience wrapper: faebryk.libs.kicad.fileformats.kicad wraps these modules and provides kicad.loads(...)/kicad.dumps(...).

Development Workflow

1. Modify Zig:
   • parsing/formatting: src/faebryk/core/zig/src/sexp/*
   • Python exposure: src/faebryk/core/zig/src/python/sexp/sexp_py.zig
2. Rebuild:
   • ato dev compile (imports faebryk.core.zig)
3. If you changed the API:
   • verify stubs under src/faebryk/core/zig/gen/sexp/*.pyi update accordingly
   • adjust src/faebryk/libs/kicad/fileformats.py if needed

Testing

• Best practical test is round-trip:
  • load a known .kicad_pcb / .kicad_sch, dump it, and ensure KiCad accepts it (formatting-sensitive).
• Zig unit tests (where present):
  • zig test src/faebryk/core/zig/src/sexp/ast.zig
  • zig test src/faebryk/core/zig/src/sexp/structure.zig

Best Practices

• Prefer faebryk.libs.kicad.fileformats.kicad unless you explicitly need the raw module API.
• Be mindful of shared-object caching in kicad.loads(...): path-based loads are cached and returned by reference (mutations are shared).

Memory & Lifetime Invariants (critical)

The Python bindings duplicate the input S-expression string into a persistent allocator because parsed structs contain pointers into the input buffer.

Implications:

• Repeated loads(...) of large files can grow memory if you never call free(...) on the returned *File.
• The convenience wrapper currently caches loaded objects by path; do not free(...) cached objects unless you also invalidate the cache.

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