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。

## How to fix:

**NOTE**:  有直达外网VPN的话，直接开启VPN就可以，一行代码都不用改。

官方文档给出的解决方案是定制文档响应函数Custom Docs UI Static Assets (Self-Hosting) - FastAPI

from fastapi import FastAPI
from fastapi.openapi.docs import (
    get_redoc_html,
    get_swagger_ui_html,
    get_swagger_ui_oauth2_redirect_html,
)

app = FastAPI(docs_url=None, redoc_url=None)


@app.get("/docs", include_in_schema=False)
async def custom_swagger_ui_html():
    return get_swagger_ui_html(
        openapi_url=app.openapi_url,
        title=app.title + " - Swagger UI",
        oauth2_redirect_url=app.swagger_ui_oauth2_redirect_url,
        swagger_js_url="https://unpkg.com/swagger-ui-dist@5/swagger-ui-bundle.js",
        swagger_css_url="https://unpkg.com/swagger-ui-dist@5/swagger-ui.css",
    )


@app.get(app.swagger_ui_oauth2_redirect_url, include_in_schema=False)
async def swagger_ui_redirect():
    return get_swagger_ui_oauth2_redirect_html()


@app.get("/redoc", include_in_schema=False)
async def redoc_html():
    return get_redoc_html(
        openapi_url=app.openapi_url,
        title=app.title + " - ReDoc",
        redoc_js_url="https://unpkg.com/redoc@next/bundles/redoc.standalone.js",
    )


@app.get("/users/{username}")
async def read_user(username: str):
    return {"message": f"Hello {username}"}

这虽然能解决问题，但会导致每次创建新项目，都需要拷贝这些相似的代码，不够pythonic

## More efficient

花了好几天的时间，写了一个开源库fastapi-cdn-host，把它变成一行代码就能搞定的：

# pip install fastapi_cdn_host
import fastapi_cdn_host
from fastapi import FastAPI


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

注：如果是要支持离线文档，只需在main.py的同级目录下，放置static文件夹（里面需包含swagger-ui-bundle.js和swagger-ui.css），启动时就会自动挂载并使用它。BTW：0.7以上版本可以直接用fastcdn offline下载swagger-ui和redoc文件到本地。

BTW：写这个库本身没花多少时间，但是写它的单元测试，真真是花了很多很多时间。由于实际测试的场景也不是很多，难免会有纰漏，欢迎来提issue: https://github.com/waketzheng/fastapi-cdn-host

> 仅支持0.100.0以上版本的fastapi，老版本的请直接用官网示例。 

## 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())