name: python-bindings-patterns description: "Instructions for python bindings patterns."

priority: critical

Python Bindings Patterns

Role: Python bindings for Rust core. Work on PyO3 bridge and Python wrapper packages.

Scope: PyO3 FFI, Python-idiomatic API, Python-specific extensions, postprocessors.

Commands: maturin develop, pytest, ruff format/check.

Critical: Core logic lives in Rust. Only Python code for bindings, Python-specific extensions, or API wrappers. If core logic needed, coordinate with Rust team.

Principles: Function-based tests only, 95% coverage, builtin imports at top, no docstrings in private/test files.

Python Modern & Performance Standards

Python 3.10+ - Functional-first - msgspec - Fully async - Strongest typing

• Target Python 3.10+; match/case, union types (X | Y), structural pattern matching
• msgspec ONLY (NEVER pydantic); msgspec.Struct with slots=True, kw_only=True, frozen=True
• Full type hints: ParamSpec for decorators, TypeVar/Generic[T], Protocol for structural typing
• Enable mypy --strict --warn-unreachable --disallow-any-expr; never use Any
• Functional patterns: pure functions, composition, map/filter/reduce, immutability
• Walrus operator := in comprehensions; match/case for conditionals
• contextlib.suppress for intentional exception suppression
• O(1) optimization: dict/set lookups over if/elif chains
• Fully async: anyio.Path (not pathlib), httpx AsyncClient, asyncpg, asyncio.gather
• Function-based tests ONLY (*_test.py); pytest fixtures, 95% coverage, real PostgreSQL
• Never: class tests, pydantic, sync I/O in async, Any type, Optional[T] (use T | None)

PyO3 Performance Patterns

Use pyo3_async_runtimes for async Python callbacks (~28x faster than spawn_blocking for fast ops).

Pattern: Check __await__ attribute, use pyo3_async_runtimes::tokio::into_future() for async, fallback to spawn_blocking for sync. Release GIL before awaiting. Use Python::attach() not with_gil().

spawn_blocking for long ops (OCR), block_in_place for quick ops (PostProcessor/Validator). CRITICAL: spawn_blocking on PostProcessor/Validator causes GIL deadlocks.