FastAPI Presentation Layer (DDD / Onion Architecture)

This skill focuses on the Presentation layer only: FastAPI routes/handlers, Pydantic schemas, and HTTP error mapping. It assumes you already have Domain + UseCase layers and repository wiring.

Non-negotiables (Onion rule)

• Presentation is the outermost layer: it can depend on UseCase and Domain types, but Domain must not depend on FastAPI/Pydantic.
• Keep business rules inside Domain/UseCase. Presentation does:

- request parsing/validation (shape, basic constraints) - conversion primitives → Value Objects - calling usecase.execute(...) - mapping Domain exceptions → HTTPException - conversion Entity → response schema

Recommended structure (per aggregate)

presentation/
  api/
    {aggregate}/
      handlers/
      schemas/
      error_messages/

Implementation checklist (per endpoint)

1. Depend on UseCase interface via Depends(get*usecase).
2. Convert inputs (UUID, str, etc.) into Domain Value Objects.
3. Handle ValueError (from Value Objects) as 400 Bad Request.
4. Execute the use case.
5. Map Domain exceptions (e.g., NotFound, lifecycle errors) to 404/400.
6. Return response model using Schema.from_entity(entity) (or equivalent).
7. Document errors in OpenAPI using responses={...: {'model': ...}}.

Route handler pattern (based on dddpy)

Prefer a small “route registrar” class per aggregate.

class TodoApiRouteHandler:
    def register_routes(self, app: FastAPI):
        @app.post("/todos", response_model=TodoSchema, status_code=201)
        def create_todo(
            data: TodoCreateSchema,
            usecase: CreateTodoUseCase = Depends(get_create_todo_usecase),
        ):
            try:
                title = TodoTitle(data.title)
                description = (
                    TodoDescription(data.description) if data.description else None
                )
            except ValueError as e:
                raise HTTPException(status_code=400, detail=str(e)) from e

            todo = usecase.execute(title, description)
            return TodoSchema.from_entity(todo)

Pydantic schemas

• Request schemas: validate shape + basic constraints (min/max length, optional fields).
• Response schemas: provide from_entity() to convert Domain types (UUID/datetime) into JSON-friendly primitives (e.g., timestamps as milliseconds).

For detailed templates and a fuller walk-through, read references/PRESENTATION.md.