```

由于我们必须用 `Query(default=None)` 替换默认值 `None`，`Query` 的第一个参数同样也是用于定义默认值。

所以：

```Python
q: Union[str, None] = Query(default=None)
```

...使得参数可选，等同于：

```Python
q: str = None
```

但是 `Query` 显式地将其声明为查询参数。

然后，我们可以将更多的参数传递给 `Query`。在本例中，适用于字符串的 `max_length` 参数：

```Python
q: Union[str, None] = Query(default=None, max_length=50)
```

        "item": {
            "name": "Foo",
            "price": 50.5,
            "description": None,
            "tax": None,
        },
        "user": {"username": "Dave", "full_name": None},
    }


@needs_py310
def test_post_body_no_data(client: TestClient):
    response = client.put("/items/5", json=None)
    assert response.status_code == 422
    assert response.json() == IsDict(
        {

                    "loc": ["body", "file"],
                    "msg": "Field required",
                    "input": None,
                },
                {
                    "type": "missing",
                    "loc": ["body", "fileb"],
                    "msg": "Field required",
                    "input": None,
                },
                {
                    "type": "missing",

    return client


@needs_py39
def test_create_item(client: TestClient) -> None:
    response = client.post("/items/", json={"name": "Foo"})
    assert response.status_code == 200, response.text
    assert response.json() == {"name": "Foo", "description": None}


@needs_py39
def test_read_items(client: TestClient) -> None:
    response = client.get("/items/")
    assert response.status_code == 200, response.text

from typing import Annotated

from fastapi import Body, FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None


@app.put("/items/{item_id}")
async def update_item(
    item_id: int,
    item: Annotated[
        Item,
        Body(
            examples=[
                {

from typing import Union

from fastapi import Body, FastAPI
from pydantic import BaseModel

app = FastAPI()


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


@app.put("/items/{item_id}")
async def update_item(
    item_id: int,
    item: Item = Body(
        examples=[
            {
                "name": "Foo",

                    "msg": "Field required",
                    "input": None,
                },
                {
                    "type": "missing",
                    "loc": ["header", "x-token"],
                    "msg": "Field required",
                    "input": None,
                },
            ]
        }
    ) | IsDict(

                    "msg": "Field required",
                    "input": None,
                },
                {
                    "type": "missing",
                    "loc": ["header", "x-token"],
                    "msg": "Field required",
                    "input": None,
                },
            ]
        }
    ) | IsDict(

from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated

app = FastAPI()


def query_extractor(q: Union[str, None] = None):
    return q


def query_or_cookie_extractor(
    q: Annotated[str, Depends(query_extractor)],
    last_query: Annotated[Union[str, None], Cookie()] = None,
):
    if not q:
        return last_query
    return q


@app.get("/items/")
async def read_query(

            "axisLabel": "",
            "axisPlacement": "auto",
            "barAlignment": 0,
            "drawStyle": "line",
            "fillOpacity": 10,
            "gradientMode": "none",
            "hideFrom": {
              "legend": false,
              "tooltip": false,
              "viz": false
            },
            "insertNulls": false,
            "lineInterpolation": "linear",