fastapi-patterns

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When applied, it prepends a system prompt before your request is sent — no extra calls and no change to how you are billed beyond the added tokens.

---
name: fastapi-patterns
description: FastAPI best practices covering project structure, Pydantic v2 schemas, dependency injection, async handlers, authentication, authorization, transactional service layers, and testing with httpx and pytest.
origin: ECC
---

# FastAPI Patterns

Modern, production-grade FastAPI development: project layout, Pydantic v2 schemas, dependency injection, async patterns, auth, transactional service methods, and testing.

## Project Structure

```text
my_app/
|-- app/
|   |-- main.py               # App factory, lifespan, middleware
|   |-- config.py             # Settings via pydantic-settings
|   |-- dependencies.py       # Shared FastAPI dependencies
|   |-- database.py           # SQLAlchemy engine + session
|   |-- routers/
|   |   `-- users.py
|   |-- models/               # SQLAlchemy ORM models
|   |   `-- user.py
|   |-- schemas/              # Pydantic request/response schemas
|   |   `-- user.py
|   `-- services/             # Business logic layer
|       `-- user_service.py
|-- tests/
|   |-- conftest.py
|   `-- test_users.py
|-- pyproject.toml
`-- .env
```

---

## App Factory and Lifespan

```python
# app/main.py
from contextlib import asynccontextmanager
from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

from app.config import settings
from app.database import engine, Base
from app.routers import users


@asynccontextmanager
async def lifespan(app: FastAPI):
    # Automatically create tables on startup for ease of use in dev/demo environments.
    # For strict production applications, manage schemas via Alembic migrations instead.
    async with engine.begin() as conn:
        await conn.run_sync(Base.metadata.create_all)
    yield
    # Shutdown: close pooled resources.
    await engine.dispose()


def create_app() -> FastAPI:
    app = FastAPI(
        title=settings.app_name,
        version=settings.app_version,
        lifespan=lifespan,
    )

    app.add_middleware(
        CORSMiddlewar

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{
  "model": "gpt-4o-mini",
  "skill": "imp-ce0826d0-58ea-49b4-8c43-7ce37badc229",
  "messages": [{ "role": "user", "content": "…" }]
}

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