# 运行时错误：请求的资源上不存在'Access-Control-Allow-Origin'头

- **ID:** `python/starlette-cors-error`
- **领域:** python
- **类别:** config_error
- **验证级别:** ai_generated
- **修复率:** 80%

## 根因

Starlette应用未配置CORS中间件，因此跨域请求被浏览器阻止。

## 版本兼容性

| 版本 | 状态 | 引入 | 弃用 |
|------|------|------|------|
| 3.x | active | — | — |

## 解决方案

1. **Add CORSMiddleware from Starlette** (95% 成功率)
   ```
   from starlette.middleware.cors import CORSMiddleware
app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_methods=["*"],
    allow_headers=["*"],
)
   ```
2. **Use FastAPI's built-in CORS middleware if using FastAPI** (90% 成功率)
   ```
   from fastapi.middleware.cors import CORSMiddleware
app.add_middleware(CORSMiddleware, allow_origins=["*"])
   ```

## 无效尝试

- **Adding CORS headers manually in the response** — Doesn't handle preflight OPTIONS requests, causing browser to block the actual request. (75% 失败率)
- **Disabling CORS check in browser** — Not a production solution; only works for local testing. (90% 失败率)