Python

The skill

Write idiomatic, type-safe Python: errors as values (Result/typed errors) and composing them, parse-don't-validate with pydantic, frozen kw-only dataclass value objects, unions with match and assert_never (structural pattern matching), Protocol vs ABC ports, persistence-ignorant domain models, package layout, import discipline and circular imports, config parsed once at a composition root that is a scope (DI/bootstrap), structured concurrency with asyncio task groups and retries, smart constructors, which interpreter and library versions to target, basedpyright, and the Any/cast/type-ignore discipline. Use when designing or reviewing Python specifically. Routing: architecture for agnostic design, mechanical-enforcement for lint and type-checker config, testing for test strategy.

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