app = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: Union[bool, None] = None


@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}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):

`Optional[Something]` là một cách viết ngắn gọn của `Union[Something, None]`, chúng là tương đương nhau.

Điều này cũng có nghĩa là trong Python 3.10, bạn có thể sử dụng `Something | None`:

//// tab | Python 3.10+

```Python hl_lines="1"
{!> ../../docs_src/python_types/tutorial009_py310.py!}
```

</div>

This means that it will **always have a value**, it's just that sometimes the value could be `None` (or `null` in JSON).

That means that, clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.

The way to describe this in OpenAPI, is to mark that field as **required**, because it will always be there.

    maxIdleConnections: Int = 5,
    keepAliveDuration: Long = 5,
    timeUnit: TimeUnit = TimeUnit.MINUTES,
    taskRunner: TaskRunner = TaskRunner.INSTANCE,
    connectionListener: ConnectionListener = ConnectionListener.NONE,
    readTimeoutMillis: Int = 10_000,
    writeTimeoutMillis: Int = 10_000,
    socketConnectTimeoutMillis: Int = 10_000,
    socketReadTimeoutMillis: Int = 10_000,
    pingIntervalMillis: Int = 10_000,

🎏 🕐❔ 📣 🔢 🔢, 🕐❔ 🏷 🔢 ✔️ 🔢 💲, ⚫️ 🚫 ✔. ⏪, ⚫️ ✔. ⚙️ `None` ⚒ ⚫️ 📦.

🖼, 👉 🏷 🔛 📣 🎻 "`object`" (⚖️ 🐍 `dict`) 💖:

```JSON
{
    "name": "Foo",
    "description": "An optional description",
    "price": 45.2,
    "tax": 3.5
}
```

... `description` & `tax` 📦 (⏮️ 🔢 💲 `None`), 👉 🎻 "`object`" 🔜 ☑:

```JSON
{
    "name": "Foo",
    "price": 45.2
}
```

Tu modelo de response podría tener valores por defecto, como:

{* ../../docs_src/response_model/tutorial004_py310.py hl[9,11:12] *}

* `description: Union[str, None] = None` (o `str | None = None` en Python 3.10) tiene un valor por defecto de `None`.
* `tax: float = 10.5` tiene un valor por defecto de `10.5`.
* `tags: List[str] = []` tiene un valor por defecto de una lista vacía: `[]`.

		length = prefixSize + 1
	} else if name[length-1] == '/' {
		length--
	}

	i := strings.LastIndex(name[:length], "/")
	nlen := len(name) - i - 1 // nlen is length of suffix
	plen := i                 // plen is length of prefix
	if i <= 0 || nlen > nameSize || nlen == 0 || plen > prefixSize {
		return "", "", false
	}
	return name[:i], name[i+1:], true
}

http://127.0.0.1:8000/items/?skip=20
```

🔢 💲 👆 🔢 🔜:

* `skip=20`: ↩️ 👆 ⚒ ⚫️ 📛
* `limit=10`: ↩️ 👈 🔢 💲

## 📦 🔢

🎏 🌌, 👆 💪 📣 📦 🔢 🔢, ⚒ 👫 🔢 `None`:

{* ../../docs_src/query_params/tutorial002.py hl[9] *}

👉 💼, 🔢 🔢 `q` 🔜 📦, & 🔜 `None` 🔢.

/// check

👀 👈 **FastAPI** 🙃 🥃 👀 👈 ➡ 🔢 `item_id` ➡ 🔢 & `q` 🚫,, ⚫️ 🔢 🔢.

///

## 🔢 🔢 🆎 🛠️

* `limit=10`: çünkü varsayılan değer buydu.

## İsteğe Bağlı Parametreler

Aynı şekilde, varsayılan değerlerini `None` olarak atayarak isteğe bağlı parametreler tanımlayabilirsiniz:

{* ../../docs_src/query_params/tutorial002_py310.py hl[7] *}

Bu durumda, `q` fonksiyon parametresi isteğe bağlı olacak ve varsayılan değer olarak `None` alacaktır.

/// check | Ek bilgi

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: Union[bool, None] = None


@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}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):