*
     * @param value The configuration value to preprocess, must not be {@code null}.
     * @param type The target type of the value, may be {@code null}.
     * @return The processed configuration value or {@code null} if none.
     * @throws BeanConfigurationException If an error occurred while preprocessing the value.
     */
    Object preprocessValue(String value, Class<?> type) throws BeanConfigurationException;

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):

from typing import Any

from fastapi import FastAPI, Response
from fastapi.testclient import TestClient

app = FastAPI()


@app.delete(
    "/{id}",
    status_code=204,
    response_model=None,
)
async def delete_deployment(
    id: int,
    response: Response,
) -> Any:
    response.status_code = 400
    return {"msg": "Status overwritten", "id": id}


client = TestClient(app)

    tokenUrl="/token",
    description="OAuth2PasswordBearer security scheme",
    auto_error=False,
)


@app.get("/items/")
async def read_items(token: Optional[str] = Security(oauth2_scheme)):
    if token is None:
        return {"msg": "Create an account first"}
    return {"token": token}


client = TestClient(app)


def test_no_token():
    response = client.get("/items")

 *  </tr>
 *  <tr>
 *      <td>implementation</td>
 *      <td>none (detected)</td>
 *      <td>Yes</td>
 *      <td>The Mojo's fully-qualified class name (or script path in the case of non-Java Mojos).</td>
 *  </tr>
 *  <tr>
 *      <td>language</td>
 *      <td>none (detected)</td>
 *      <td>No. Default: <code>java</code></td>

import orjson
from fastapi import FastAPI, Response

app = FastAPI()


class CustomORJSONResponse(Response):
    media_type = "application/json"

    def render(self, content: Any) -> bytes:
        assert orjson is not None, "orjson must be installed"
        return orjson.dumps(content, option=orjson.OPT_INDENT_2)


@app.get("/", response_class=CustomORJSONResponse)
async def main():

    assert response.status_code == 200, response.text
    assert response.json() == {
        "name": "Foo",
        "price": 42,
        "description": None,
        "tax": None,
        "tags": [],
    }


@needs_py310
@needs_pydanticv2
def test_openapi_schema(client: TestClient):
    response = client.get("/openapi.json")

    assert response.json() == {
        "name": "Island In The Moon",
        "price": 12.99,
        "description": "A place to be playin' and havin' fun",
        "tags": ["breater"],
        "tax": None,
    }


def test_openapi_schema():
    response = client.get("/openapi.json")
    assert response.status_code == 200
    assert response.json() == {
        "openapi": "3.1.0",

def test_query_no_values():
    url = "/items/"
    response = client.get(url)
    assert response.status_code == 200, response.text
    assert response.json() == {"q": None}


def test_openapi_schema():
    response = client.get("/openapi.json")
    assert response.status_code == 200, response.text
    assert response.json() == {
        "openapi": "3.1.0",

## 响应模型编码参数

你的响应模型可以具有默认值，例如：

```Python hl_lines="11  13-14"
{!../../docs_src/response_model/tutorial004.py!}
```

* `description: Union[str, None] = None` 具有默认值 `None`。
* `tax: float = 10.5` 具有默认值 `10.5`.
* `tags: List[str] = []` 具有一个空列表作为默认值： `[]`.

但如果它们并没有存储实际的值，你可能想从结果中忽略它们的默认值。

举个例子，当你在 NoSQL 数据库中保存了具有许多可选属性的模型，但你又不想发送充满默认值的很长的 JSON 响应。