与声明查询参数一样，包含默认值的模型属性是可选的，否则就是必选的。默认值为 `None` 的模型属性也是可选的。

例如，上述模型声明如下 JSON **对象**（即 Python **字典**）：

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

……由于 `description` 和 `tax` 是可选的（默认值为 `None`），下面的 JSON **对象**也有效：

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

## 声明请求体参数

        "paths": {
            "/items/": {
                "get": {
                    "responses": {
                        "200": {
                            "description": "Successful Response",
                            "content": {"application/json": {"schema": {}}},
                        }
                    },
                    "summary": "Read Items",

                "get": {
                    "summary": "Main",
                    "operationId": "main__get",
                    "responses": {
                        "200": {
                            "description": "Successful Response",
                            "content": {"text/plain": {"schema": {"type": "string"}}},
                        }
                    },
                }
            }
        },

  <groupId>inheritance</groupId>
  <artifactId>inheritance</artifactId>
  <version>11-SNAPSHOT</version>
  <name>Model urls inheritance test child</name>
  <description>Flat directory structure case: module = ../child-artifact-id + child directory path == child-artifact-id</description>

  <!-- 5 inherited urls with ../${project.artifactId} added to parent -->
  <url>http://www.apache.org/path/to/parent/../inheritance/</url>
  <scm>

  <groupId>inheritance</groupId>
  <artifactId>parent</artifactId>
  <version>11-SNAPSHOT</version>

  <name>Model urls inheritance test parent</name>
  <description>Flat directory structure case: module = ../child-artifact-id + child directory path == child-artifact-id</description>

  <modules>
    <module>../inheritance</module>
  </modules>

  <!-- 5 urls in the pom will be inherited with path added -->

  <groupId>inheritance</groupId>
  <artifactId>parent</artifactId>
  <version>11-SNAPSHOT</version>

  <name>Model urls inheritance test parent</name>
  <description>Most classical case: child in direct subdirectory with directory name == child artifactId</description>

  <modules>
    <module>child-artifact-id</module>
  </modules>

  <!-- 5 urls in the pom will be inherited with path added -->

from typing import Annotated, 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


class User(BaseModel):
    username: str
    full_name: Union[str, None] = None


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

                                "application/json": {
                                    "schema": {},
                                },
                            },
                            "description": "Successful Response",
                        },
                    },
                    "summary": "Read Users2",
                },
            },
        },

                && options.model().isEmpty()) {
            return true;
        }

        return false;
    }

    /**
     * Gets a description of what this strategy does.
     *
     * @return a human-readable description of the strategy
     */
    String getDescription();

    id: str
    value: str


app = FastAPI()


@app.get(
    "/items/{item_id}",
    response_model=Item,
    responses={
        200: {
            "content": {"image/png": {}},
            "description": "Return the JSON item or an image.",
        }
    },
)
async def read_item(item_id: str, img: Union[bool, None] = None):
    if img:
        return FileResponse("image.png", media_type="image/png")
    else: