{* ../../docs_src/separate_openapi_schemas/tutorial001_py310.py ln[1:15] hl[14] *}

...then the `description` field will **not be required**. Because it has a default value of `None`.

### Input Model in Docs { #input-model-in-docs }

You can confirm that in the docs, the `description` field doesn't have a **red asterisk**, it's not marked as required:

<div class="screenshot">

在 OpenAPI 中，描述這種情況的方式是將該欄位標記為必填，因為它一定存在。

因此，同一個模型的 JSON Schema 會依用於輸入或輸出而不同：

- 用於輸入時，`description` 不是必填
- 用於輸出時，`description` 是必填（且可能為 `None`，在 JSON 中為 `null`）

### 文件中的輸出模型 { #model-for-output-in-docs }

你也可以在文件中檢視輸出模型，`name` 與 `description` 都以紅色星號標示為必填：

<div class="screenshot">
<img src="/img/tutorial/separate-openapi-schemas/image03.png">
</div>

			Description: "Number of events per HTTP send to webhook target",
			Optional:    true,
			Type:        "number",
		},
		config.HelpKV{
			Key:         QueueSize,
			Description: "configure channel queue size for webhook targets",
			Optional:    true,
			Type:        "number",
		},
		config.HelpKV{
			Key:         QueueDir,
			Description: `staging dir for undelivered logger messages e.g. '/home/logger-events'`,
			Optional:    true,

          <name>description</name>
          <description>The description of what this expression references, and what it's generally used for.</description>
          <type>String</type>
          <required>true</required>
        </field>
        <field>
          <version>1.0.0</version>
          <name>configuration</name>

			Key:         config.Comment,
			Description: config.DefaultComment,
			Optional:    true,
			Type:        "sentence",
		},
		config.HelpKV{
			Key:         target.WebhookClientCert,
			Description: "client cert for Webhook mTLS auth",
			Optional:    true,
			Type:        "string",
			Sensitive:   true,
		},
		config.HelpKV{
			Key:         target.WebhookClientKey,
			Description: "client cert key for Webhook mTLS auth",

	}

	Help = config.HelpKVS{
		config.HelpKV{
			Key:         browserCSPPolicy,
			Description: `set Content-Security-Policy response header value` + defaultHelpPostfix(browserCSPPolicy),
			Optional:    true,
			Type:        "string",
		},
		config.HelpKV{
			Key:         browserHSTSSeconds,
			Description: `set Strict-Transport-Security 'max-age' amount of seconds value` + defaultHelpPostfix(browserHSTSSeconds),

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float = 10.5


items = {
    "foo": {"name": "Foo", "price": 50.2},
    "bar": {"name": "Bar", "description": "The Bar fighters", "price": 62, "tax": 20.2},
    "baz": {
        "name": "Baz",
        "description": "There goes my baz",
        "price": 50.2,
        "tax": 10.5,
    },
}

          <version>3.0.0+</version>
          <required>true</required>
          <description>The full name of the project.</description>
          <type>String</type>
        </field>
        <field>
          <name>description</name>
          <version>3.0.0+</version>
          <description>A detailed description of the project, used by Maven whenever it needs to

### 输入用的模型 { #model-for-input }

如果你像下面这样把该模型用作输入：

{* ../../docs_src/separate_openapi_schemas/tutorial001_py310.py ln[1:15] hl[14] *}

...那么 `description` 字段将**不是必填项**，因为它的默认值是 `None`。

### 文档中的输入模型 { #input-model-in-docs }

你可以在文档中确认，`description` 字段没有**红色星号**，也就是未被标记为必填：

<div class="screenshot">
<img src="/img/tutorial/separate-openapi-schemas/image01.png">
</div>

### 输出用的模型 { #model-for-output }

from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None


items = [
    Item(name="Plumbus", description="A multi-purpose household device."),
    Item(name="Portal Gun", description="A portal opening device."),
    Item(name="Meeseeks Box", description="A box that summons a Meeseeks."),
]


@app.get("/items/stream")