fastapi-patterns

FastAPI 模式

使用时机

在以下情况下启用此技能：

• 配置 FastAPI 中间件（CORS、日志记录、计时、错误处理）
• 创建复杂的依赖注入链
• 实现带连接管理的 WebSocket 端点
• 自定义 OpenAPI 文档（标签、示例、弃用标记）
• 设置 JWT 身份验证和基于角色的访问依赖项
• 实现后台任务（轻量级或分布式）
• 管理应用生命周期（通过 lifespan 处理启动和关闭）
• 设置速率限制或请求节流

不要在以下情况下使用此技能：

• 基本的端点 CRUD、仓储或服务模式（请使用 python-backend-expert）
• 为 FastAPI 端点编写测试（请使用 pytest-patterns）
• API 契约设计或架构规划（请使用 api-design-patterns）
• 架构决策（请使用 system-architecture）

说明

中间件栈

中间件按 LIFO（后进先出）顺序执行。最后添加的中间件位于最外层。

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

# Order matters: added last = executed first (outermost)
app.add_middleware(TimingMiddleware)          # 3rd added = runs 1st
app.add_middleware(RequestLoggingMiddleware)  # 2nd added = runs 2nd
app.add_middleware(                           # 1st added = runs 3rd (innermost)
    CORSMiddleware,
    allow_origins=["https://app.example.com"],  # NEVER use "*" in production
    allow_credentials=True,
    allow_methods=["GET", "POST", "PUT", "PATCH", "DELETE"],
    allow_headers=["Authorization", "Content-Type"],
)

ASGI 中间件（首选）

对于性能关键路径，使用纯 ASGI 中间件：

from starlette.types import ASGIApp, Receive, Scope, Send
import time

class TimingMiddleware:
    def __init__(self, app: ASGIApp) -> None:
        self.app = app

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        start = time.perf_counter()
        await self.app(scope, receive, send)
        duration = time.perf_counter() - start
        # Log or record the duration

BaseHTTPMiddleware（更简单，但速度较慢）

仅在中间件需要读取或修改请求体或响应时使用：

from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request
from starlette.responses import Response

class RequestIdMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next) -> Response:
        request_id = request.headers.get("X-Request-ID", str(uuid.uuid4()))
        request.state.request_id = request_id
        response = await call_next(request)
        response.headers["X-Request-ID"] = request_id
        return response

如何选择：

• ASGI 中间件： 性能关键，且无需读取请求体或响应体
• BaseHTTPMiddleware： 需要访问 Request/Response 对象，API 更简单

身份验证依赖项

JWT 令牌验证

from fastapi import Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer
from jose import JWTError, jwt

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/auth/login")

async def get_current_user(
    token: str = Depends(oauth2_scheme),
    session: AsyncSession = Depends(get_async_session),
) -> User:
    try:
        payload = jwt.decode(token, settings.jwt_secret, algorithms=["HS256"])
        user_id: int = payload.get("sub")
        if user_id is None:
            raise HTTPException(status_code=401, detail="Invalid token")
    except JWTError:
        raise HTTPException(status_code=401, detail="Invalid token")

    user = await session.get(User, user_id)
    if user is None or not user.is_active:
        raise HTTPException(status_code=401, detail="User not found or inactive")
    return user

基于角色的访问控制（工厂模式）

def require_role(*roles: str):
    """Factory that creates a dependency requiring specific roles."""
    async def check_role(user: User = Depends(get_current_user)) -> User:
        if user.role not in roles:
            raise HTTPException(
                status_code=403,
                detail=f"Requires one of: {', '.join(roles)}",
            )
        return user
    return check_role

# Usage in routes
@router.delete("/users/{user_id}", dependencies=[Depends(require_role("admin"))])
async def delete_user(user_id: int, ...) -> None:
    ...

@router.patch("/posts/{post_id}")
async def update_post(
    post_id: int,
    user: User = Depends(require_role("admin", "editor")),
) -> PostResponse:
    ...

依赖注入链

缓存行为

FastAPI 会在单个请求中缓存依赖项的结果。同一依赖项被多次调用时，会返回同一个实例：

# get_async_session is called once per request, even if used by multiple deps
async def get_user_service(session: AsyncSession = Depends(get_async_session)) -> UserService:
    return UserService(session)

async def get_post_service(session: AsyncSession = Depends(get_async_session)) -> PostService:
    return PostService(session)  # Same session instance as user_service

要禁用缓存（每次都获取新实例），请使用 use_cache=False：

session: AsyncSession = Depends(get_async_session, use_cache=False)

Yield 依赖项（资源清理）

async def get_http_client() -> AsyncGenerator[httpx.AsyncClient, None]:
    async with httpx.AsyncClient(timeout=30.0) as client:
        yield client
    # Client is automatically closed after the request

在测试中覆盖依赖项

# In tests
from app.main import app
from app.dependencies.auth import get_current_user

async def mock_current_user() -> User:
    return User(id=1, email="[email protected]", role="admin")

app.dependency_overrides[get_current_user] = mock_current_user

后台任务

FastAPI BackgroundTasks（轻量级）

对于无需在服务重启后继续执行的任务：

from fastapi import BackgroundTasks

@router.post("/users", status_code=201)
async def create_user(
    data: UserCreate,
    background_tasks: BackgroundTasks,
    service: UserService = Depends(get_user_service),
) -> UserResponse:
    user = await service.create_user(data)
    background_tasks.add_task(send_welcome_email, user.email)
    return UserResponse.model_validate(user)

async def send_welcome_email(email: str) -> None:
    """Runs after the response is sent. Creates its own session."""
    async with async_session_factory() as session:
        async with session.begin():
            # Send email, log activity, etc.
            ...

规则：

• 切勿在后台任务中复用请求会话——请创建新会话
• 后台任务在同一进程中运行——不重试、不持久化
• 对于需要可靠性、重试或分布式执行的任务，请使用 Celery 或类似工具

WebSocket 模式

from fastapi import WebSocket, WebSocketDisconnect

class ConnectionManager:
    def __init__(self) -> None:
        self.active: dict[int, list[WebSocket]] = {}

    async def connect(self, user_id: int, ws: WebSocket) -> None:
        await ws.accept()
        self.active.setdefault(user_id, []).append(ws)

    def disconnect(self, user_id: int, ws: WebSocket) -> None:
        if user_id in self.active:
            self.active[user_id].remove(ws)
            if not self.active[user_id]:
                del self.active[user_id]

    async def send_to_user(self, user_id: int, message: dict) -> None:
        for ws in self.active.get(user_id, []):
            await ws.send_json(message)

manager = ConnectionManager()

@router.websocket("/ws")
async def websocket_endpoint(ws: WebSocket, token: str) -> None:
    # Auth via query parameter: /ws?token=xxx
    user = await verify_ws_token(token)
    if not user:
        await ws.close(code=4001)
        return

    await manager.connect(user.id, ws)
    try:
        while True:
            data = await ws.receive_json()
            # Process incoming messages
            await handle_message(user.id, data)
    except WebSocketDisconnect:
        manager.disconnect(user.id, ws)

WebSocket 身份验证方式：

1. 查询参数：ws://host/ws?token=xxx（最简单，但令牌会显示在日志中）
2. 首条消息：连接后将令牌作为首条消息发送（更安全）
3. Cookie：使用现有的会话 Cookie（要求域名相同）

应用生命周期

使用 lifespan 上下文管理器（不要使用已弃用的 on_event）：

from contextlib import asynccontextmanager
from collections.abc import AsyncIterator
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI) -> AsyncIterator[None]:
    # Startup: initialize resources
    await init_database()
    redis = await aioredis.from_url(settings.redis_url)
    app.state.redis = redis

    yield  # Application runs here

    # Shutdown: cleanup resources
    await redis.close()
    await dispose_engine()

app = FastAPI(lifespan=lifespan)

生命周期职责：

• 初始化和释放数据库连接池
• 设置和拆除 Redis/缓存连接
• 创建 HTTP 客户端连接池
• 启动和关闭后台调度器
• 启动时预热缓存

异常处理器

注册全局异常处理器，以统一错误响应：

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError

@app.exception_handler(RequestValidationError)
async def validation_handler(request: Request, exc: RequestValidationError):
    return JSONResponse(
        status_code=422,
        content={
            "detail": "Validation error",
            "code": "VALIDATION_ERROR",
            "field_errors": [
                {"field": e["loc"][-1], "message": e["msg"], "code": e["type"]}
                for e in exc.errors()
            ],
        },
    )

OpenAPI 自定义

app = FastAPI(
    title="My API",
    version="1.0.0",
    description="API description with **markdown** support",
    openapi_tags=[
        {"name": "Users", "description": "User management operations"},
        {"name": "Auth", "description": "Authentication endpoints"},
    ],
    docs_url="/docs",        # Swagger UI
    redoc_url="/redoc",      # ReDoc
    openapi_url="/openapi.json",
)

示例

JWT 身份验证依赖链

从令牌到已授权用户的完整身份验证链：

Request with Authorization: Bearer <token>
    ↓
oauth2_scheme (extracts token from header)
    ↓
get_current_user (decodes JWT, loads user from DB)
    ↓
require_role("admin") (checks user.role)
    ↓
Route handler (receives verified admin user)

链中的每个依赖项都可以通过 dependency_overrides 独立测试。

边缘情况

• 中间件与依赖项： 使用中间件处理横切关注点（日志记录、计时、CORS）。使用依赖项处理路由级逻辑（身份验证、分页参数、功能标志）。

• ASGI 与 BaseHTTPMiddleware： 优先使用 ASGI 中间件以获得更好的性能。BaseHTTPMiddleware 会将整个响应正文读入内存，从而导致流式传输和大型响应出现问题。

• Lifespan 与 on_event： 始终使用 lifespan 上下文管理器。FastAPI 0.109+ 中的 @app.on_event("startup") 和 @app.on_event("shutdown") 已弃用。

• 跨子应用的 Depends 缓存： 依赖项缓存仅在单个应用实例内按请求生效。如果使用 app.mount() 挂载子应用，每个子应用都有自己的依赖项解析范围。

• WebSocket 扩展： 单个服务实例会持有所有 WebSocket 连接。对于多实例部署，请使用 Redis pub/sub 在各实例之间广播消息。

参见 references/middleware-examples.md，了解完整的中间件实现。 参见 references/dependency-injection-patterns.md，了解高级 DI 模式。