python network_error ai_generated true

从源 'http://localhost:3000' 访问 'http://localhost:8000/api' 的 fetch 已被 CORS 策略阻止：请求的资源上没有 'Access-Control-Allow-Origin' 头。

Access to fetch at 'http://localhost:8000/api' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

ID: python/fastapi-cors-preflight-fail

其他格式: JSON · Markdown 中文 · English
80%修复率
89%置信度
0证据数
2024-09-12首次发现

版本兼容性

版本状态引入弃用备注
3.x active — — —

根因分析

缺少 CORSMiddleware 或顺序错误；在 Starlette 中中间件顺序很重要，CORSMiddleware 必须包裹应用。

English

CORSMiddleware is missing or misordered; in Starlette, middleware order matters and CORSMiddleware must wrap the app.

generic

解决方案

  1. 93% 成功率 
    app.add_middleware(CORSMiddleware, allow_origins=['http://localhost:3000'], allow_methods=['*'], allow_headers=['*'])
  2. 90% 成功率 
    In Flask: CORS(app, resources={r'/api/*': {'origins': 'http://localhost:3000'}})

无效尝试

常见但无效的做法:

  1. 75% 失败

    Browsers reject wildcard with credentials.

  2. 70% 失败

    Preflight OPTIONS still not handled.