!!! info
    You can also use:

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    as described in <a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">the Pydantic docs</a> for `exclude_defaults` and `exclude_none`.

!!! info
    你还可以使用：

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    参考 <a href="https://docs.pydantic.dev/latest/concepts/serialization/#modeldict" class="external-link" target="_blank">Pydantic 文档</a> 中对 `exclude_defaults` 和 `exclude_none` 的描述。

#### 默认值字段有实际值的数据

!!! info "情報"
    以下も使用することができます:

    * `response_model_exclude_defaults=True`
    * `response_model_exclude_none=True`

    `PATCH` 没有 `PUT` 知名，也怎么不常用。

    很多人甚至只用 `PUT` 实现部分更新。

    **FastAPI** 对此没有任何限制，可以**随意**互换使用这两种操作。

    但本指南也会分别介绍这两种操作各自的用途。

### 使用 Pydantic 的 `exclude_unset` 参数

更新部分数据时，可以在 Pydantic 模型的 `.dict()` 中使用 `exclude_unset` 参数。

比如，`item.dict(exclude_unset=True)`。

这段代码生成的 `dict` 只包含创建 `item` 模型时显式设置的数据，而不包括默认值。

然后再用它生成一个只含已设置（在请求中所发送）数据，且省略了默认值的 `dict`：

```Python hl_lines="34"

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


@app.get("/items/{item_id}", response_model=Item, response_model_exclude_unset=True)
async def read_item(item_id: str):

    response_model_include=["name", "description"],
)
async def read_item_name(item_id: str):
    return items[item_id]


@app.get("/items/{item_id}/public", response_model=Item, response_model_exclude=["tax"])
async def read_item_public_data(item_id: str):

{!../../../docs_src/dataclasses/tutorial003.py!}
```

1. We still import `field` from standard `dataclasses`.

2. `pydantic.dataclasses` is a drop-in replacement for `dataclasses`.

3. The `Author` dataclass includes a list of `Item` dataclasses.

4. The `Author` dataclass is used as the `response_model` parameter.

5. You can use other standard type annotations with dataclasses as the request body.

```

`app.add_middleware()` receives a middleware class as the first argument and any additional arguments to be passed to the middleware.

## Integrated middlewares

**FastAPI** includes several middlewares for common use cases, we'll see next how to use them.

!!! note "Technical Details"
    For the next examples, you could also use `from starlette.middleware.something import SomethingMiddleware`.

Different versions of **FastAPI** will use a specific newer version of Starlette.

So, you can just let **FastAPI** use the correct Starlette version.

## About Pydantic

Pydantic includes the tests for **FastAPI** with its own tests, so new versions of Pydantic (above `1.0.0`) are always compatible with FastAPI.

You can pin Pydantic to any version above `1.0.0` that works for you and below `2.0.0`.

For example:

This schema definition includes your API paths, the possible parameters they take, etc.

#### Data "schema"

The term "schema" might also refer to the shape of some data, like a JSON content.

In that case, it would mean the JSON attributes, and data types they have, etc.

#### OpenAPI and JSON Schema