router_b_override = APIRouter()  # Overrides default class
router_b_a = APIRouter()
router_b_a_c_override = APIRouter()  # Overrides default class again


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


@app.get("/override", response_class=PlainTextResponse)
def get_path_override():
    return "Hello World"


@router_a.get("/")
def get_a():

## উদাহরণ

### তৈরি

- `main.py` নামে একটি ফাইল তৈরি করুন:

```Python
from typing import Union

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: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">

你还可以返回 Pydantic 模型（稍后你将了解更多）。

还有许多其他将会自动转换为 JSON 的对象和模型（包括 ORM 对象等）。尝试下使用你最喜欢的一种，它很有可能已经被支持。

## 总结

* 导入 `FastAPI`。
* 创建一个 `app` 实例。
* 编写一个**路径操作装饰器**（如 `@app.get("/")`）。
* 编写一个**路径操作函数**（如上面的 `def root(): ...`）。

### Crie o código **FastAPI**

* Crie um diretório `app` e entre nele.
* Crie um arquivo `main.py` com:

```Python
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: str = None):
    return {"item_id": item_id, "q": q}
```

## 0.56.0

* Add support for ASGI `root_path`:
    * Use `root_path` internally for mounted applications, so that OpenAPI and the docs UI works automatically without extra configurations and parameters.
    * Add new `root_path` parameter for `FastAPI` applications to provide it in cases where it can be set with the command line (e.g. for Uvicorn and Hypercorn, with the parameter `--root-path`).

* 빈 파일 `__init__.py`을 생성합니다.
* 다음과 같은 `main.py`을 생성합니다:

```Python
from typing import Union

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: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

### 도커파일

* **Fácil**: Projetado para ser fácil de aprender e usar. Menos tempo lendo documentação.
* **Enxuto**: Minimize duplicação de código. Múltiplos recursos para cada declaração de parâmetro. Menos bugs.
* **Robusto**: Tenha código pronto para produção. E com documentação interativa automática.

import pytest

from docs_src.async_tests.test_main import test_root


@pytest.mark.anyio
async def test_async_testing():

### Technische Details: `root_path`

Wenn Sie eine Unteranwendung wie oben beschrieben mounten, kümmert sich FastAPI darum, den Mount-Pfad für die Unteranwendung zu kommunizieren, mithilfe eines Mechanismus aus der ASGI-Spezifikation namens `root_path`.

from fastapi.testclient import TestClient

from docs_src.behind_a_proxy.tutorial001 import app

client = TestClient(app, root_path="/api/v1")


def test_main():
    response = client.get("/app")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello World", "root_path": "/api/v1"}


def test_openapi():
    response = client.get("/openapi.json")
    assert response.status_code == 200