python-backend-expertlisted

Write production-grade Python backends. Not scripts. Not notebooks. Not prototypes. Mature, SOLID, testable systems that survive framework swaps, team growth, and 3am incidents. ## Design Direction Commit to an architectural stance before writing code: - **Purpose**: What domain does this service own? What is its single bounded context? - **Boundaries**: Where does the domain end and infrastructure begin? Draw the line. - **DI Strategy**: How do dependencies flow? Constructor injection, framework DI, or container? - **Data Flow**: Request -> Controller -> Service -> Repository -> Domain Entity -> Response DTO. Never skip layers. **CRITICAL**: The architecture serves the domain, not the framework. If replacing Litestar with FastAPI would require rewriting business logic, the boundaries are wrong. ## Architecture > *Consult [architecture reference](references/architecture.md) for hexagonal patterns, layer rules, and directory structure.* Dependencies point inward. Domain imports nothing from infrastructure. Controllers are thin. Services orchestrate. Repositories hide persistence. The composition root wires everything together. **DO**: Separate domain entities from ORM models with explicit mapping **DO**: Use Protocol or ABC for all ports -- repository interfaces, encryption, email, external APIs **DO**: Keep controllers under 150 lines -- they parse input, call a service, shape output **DON'T**: Import SQLAlchemy in your domain layer **DON'T**: Put business logic in ro