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

查询参数的值就是：

* `skip=20`：在 URL 中设定的值
* `limit=10`：使用默认值

## 可选参数 { #optional-parameters }

同理，把默认值设为 `None` 即可声明可选的查询参数：

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

本例中，查询参数 `q` 是可选的，默认值为 `None`。

/// check | 检查

注意，**FastAPI** 可以识别出 `item_id` 是路径参数，`q` 不是路径参数，而是查询参数。

///

## 查询参数类型转换 { #query-parameter-type-conversion }

</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.

app = FastAPI()

security = HTTPBase(scheme="Other", auto_error=False)


@app.get("/users/me")
def read_current_user(
    credentials: HTTPAuthorizationCredentials | None = Security(security),
):
    if credentials is None:
        return {"msg": "Create an account first"}
    return {"scheme": credentials.scheme, "credentials": credentials.credentials}


client = TestClient(app)

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float
    is_offer: bool | None = None


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


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


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

    )
    assert response.status_code == 200
    assert response.json() == {
        "item_id": 123,
        "name": "Foo",
        "price": 50.1,
        "description": None,
        "tax": None,
    }


def test_put_with_no_data(client: TestClient):
    response = client.put("/items/123", json={})
    assert response.status_code == 422
    assert response.json() == {
        "detail": [

        "name": "Baz",
        "description": None,
        "price": 50.2,
        "tax": 10.5,
        "tags": [],
    }


def test_put(client: TestClient):
    response = client.put(
        "/items/bar", json={"name": "Barz", "price": 3, "description": None}
    )
    assert response.json() == {
        "name": "Barz",
        "description": None,
        "price": 3,
        "tax": 10.5,

Ihr Responsemodell könnte Defaultwerte haben, wie:

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

* `description: Union[str, None] = None` (oder `str | None = None` in Python 3.10) hat einen Defaultwert `None`.
* `tax: float = 10.5` hat einen Defaultwert `10.5`.
* `tags: List[str] = []` hat eine leere Liste als Defaultwert: `[]`.

/// note | Nota

O FastAPI saberá que o valor de `q` não é obrigatório por causa do valor padrão `= None`.

O `str | None` não é utilizado pelo FastAPI para determinar que o valor não é obrigatório, ele saberá que não é obrigatório porque tem um valor padrão `= None`.

Mas adicionar as anotações de tipo permitirá ao seu editor oferecer um suporte melhor e detectar erros.

///

    responses={
        200: {
            "content": {"image/png": {}},
            "description": "Return the JSON item or an image.",
        }
    },
)
async def read_item(item_id: str, img: bool | None = None):
    if img:
        return FileResponse("image.png", media_type="image/png")
    else:

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons=Depends(CommonQueryParams)):
    response = {}
    if commons.q: