python-api-endpoint-creator

Python API Endpoint Creator

Creates complete FastAPI endpoints following strict layered architecture patterns.

When to Use

• Creating a new REST API endpoint from scratch
• Adding CRUD operations for a new domain entity
• Setting up the full stack: Router → Service → Repository → Tests
• Scaffolding a new domain module

Process

1. Pydantic Schemas — Create/Update/Response in schemas.py
2. SQLAlchemy Model — with SoftDeleteMixin in models.py
3. Repository — async CRUD with soft delete in repository.py
4. Service — business logic, HTTPException for errors in service.py
5. Dependencies — get_db, get_service in dependencies.py
6. Router — thin endpoints, Depends() for everything in router.py
7. Tests — httpx AsyncClient tests in tests/

See code-patterns.md for complete templates for each file.

Architecture

Router (APIRouter)  →  Service  →  Repository  →  Database
      ↓                  ↓            ↓
  Depends()        Business logic  SQLAlchemy
  Pydantic schemas HTTPException   AsyncSession
  Status codes     Validation      Soft delete

Hard Rules

• POST for all mutations — GET for reads, POST for create/update/delete
• Query params for IDs — ?id=xxx, never path params /{id}
• Routers are thin — just delegation, no business logic
• Services raise HTTPException — for expected errors (404, 409, 422)
• Repositories handle soft delete — filter deleted_at.is_(None) in all queries
• async def for all endpoints — unless CPU-bound

Gotchas

• Forgetting response_model makes your API leak internal fields. Always set response_model=YourSchema on endpoints. Without it, FastAPI serializes the raw return value, which may include hashed_password, deleted_at, or other internal fields from your SQLAlchemy model.

• Depends() creates a new instance per request — not a singleton. If your service holds state, it will be lost between requests. Services should be stateless. If you need shared state, use app state or a cache.

• await db.commit() in the dependency, not in the repository. The get_db dependency handles commit/rollback. If you also commit inside the repository, you get double-commit bugs or premature commits before all operations complete.

• 422 errors from Pydantic are opaque by default. Override the RequestValidationError handler to return field-level errors. The default just says "validation error" with no useful detail for the frontend.

• Missing status_code=201 on create endpoints. FastAPI defaults to 200. Explicitly set status_code=status.HTTP_201_CREATED on POST create routes and status_code=status.HTTP_204_NO_CONTENT on delete routes.