如图：

原因是FastAPI的接口文档默认使用https://cdn.jsdelivr.net/npm/swagger-ui-dist@5.9.0/swagger-ui.css

和https://cdn.jsdelivr.net/npm/swagger-ui-dist@5.9.0/swagger-ui-bundle.js

来渲染页面，而这两个URL是外网的CDN，在国内响应超慢，导致请求超时了。

解决方案是更换CDN或把这两个URL对应的文件下载到本地的static目录中并挂载它，具体代码如下：

from fastapi import FastAPI
# pip install fastapi_cdn_host
from fastapi_cdn_host import patch_docs


app = FastAPI()
# 启动app时会自动将CDN更换为响应速度较快的那个
patch_docs(app)

注：如果是要支持离线文档，可以直接用这个：fastapi-offline · PyPI

## 2024.05.12 补充：一个详细一点的示例

- main.py

#!/usr/bin/env python
import os
import sys
from contextlib import asynccontextmanager
from datetime import datetime

import fastapi_cdn_host
from fastapi import FastAPI, Request
from fastapi.responses import RedirectResponse

__version__ = "1.0.0"


@asynccontextmanager
async def lifespan(app: FastAPI):
    # 注册数据库、挂载Redis等……
    yield


app = FastAPI(title="FastAPI CDN Host Demo", lifespan=lifespan, version=__version__)
fastapi_cdn_host.patch_docs(app)


@app.get("/", include_in_schema=False)
async def root(request: Request) -> RedirectResponse:
    """首页直接跳转到文档"""
    return RedirectResponse("/docs")


@app.get("/robots.txt", include_in_schema=False)
async def robots_txt():
    """声明不允许爬虫访问"""
    return """
    User-agent: *

    Disallow: /
    """


@app.get("/app")
async def app_info(request: Request) -> dict[str, str | dict | datetime | None]:
    """展示客户端IP、服务器时间等信息"""
    headers = dict(request.headers)
    ip = getattr(request.client, "host", "")
    url = request.url
    return {
        "your ip": ip,
        "version": __version__,
        "now": datetime.now(),
        "headers": headers,
        "path": request.url.path,
        "url": {"scheme": url.scheme, "hostname": url.hostname, "port": url.port},
    }


def _runserver() -> int:
    """This is for debug mode to start server. For prod, use supervisor+gunicorn instead."""
    return os.system("fastapi dev")


if __name__ == "__main__":  # pragma: no cover
    sys.exit(_runserver())