"openapi": "3.1.0",
        "info": {"title": "FastAPI", "version": "0.1.0"},
        "paths": {
            "/": {
                "get": {
                    "summary": "Read Root",
                    "operationId": "read_root__get",
                    "responses": {
                        "200": {
                            "description": "Successful Response",
                            "content": {

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():

import uvicorn
from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def root():
    a = "a"
    b = "b" + a
    return {"hello world": b}


if __name__ == "__main__":

app.include_router(
    admin.router,
    prefix="/admin",
    tags=["admin"],
    dependencies=[Depends(get_token_header)],
    responses={418: {"description": "I'm a teapot"}},
)


@app.get("/")
async def root():

        <mxGraphModel dx="2738" dy="2173" grid="1" gridSize="10" guides="1" tooltips="1" connect="1" arrows="1" fold="1" page="1" pageScale="1" pageWidth="1920" pageHeight="1200" math="0" shadow="0">
            <root>
                <mxCell id="0"/>
                <mxCell id="1" parent="0"/>

* Créer un répertoire `app` et y entrer.
* Créez un fichier `main.py` avec :

```Python
from typing import Optional

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Optional[str] = None):
    return {"item_id": item_id, "q": q}
```

📤 📚 🎏 🎚 & 🏷 👈 🔜 🔁 🗜 🎻 (🔌 🐜, ♒️). 🔄 ⚙️ 👆 💕 🕐, ⚫️ 🏆 🎲 👈 👫 ⏪ 🐕‍🦺.

## 🌃

* 🗄 `FastAPI`.
* ✍ `app` 👐.
* ✍ **➡ 🛠️ 👨‍🎨** (💖 `@app.get("/")`).
* ✍ **➡ 🛠️ 🔢** (💖 `def root(): ...` 🔛).

<img src="/img/tutorial/sub-applications/image02.png">

两个用户界面都可以正常运行，因为浏览器能够与每个指定的应用或子应用会话。

### 技术细节：`root_path`

以上述方式挂载子应用时，FastAPI 使用 ASGI 规范中的 `root_path` 机制处理挂载子应用路径之间的通信。

这样，子应用就可以为自动文档使用路径前缀。

并且子应用还可以再挂载子应用，一切都会正常运行，FastAPI 可以自动处理所有 `root_path`。

### Technical Details: `root_path`

When you mount a sub-application as described above, FastAPI will take care of communicating the mount path for the sub-application using a mechanism from the ASGI specification called a `root_path`.

That way, the sub-application will know to use that path prefix for the docs UI.

from fastapi import FastAPI, Request

app = FastAPI()


@app.get("/app")
def read_main(request: Request):