* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.
* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).

# Without aliases


@app.post("/optional-list-str", operation_id="optional_list_str")
async def read_optional_list_str(
    p: Annotated[Optional[list[str]], Body(embed=True)] = None,
):
    return {"p": p}


class BodyModelOptionalListStr(BaseModel):
    p: Optional[list[str]] = None


@app.post("/model-optional-list-str", operation_id="model_optional_list_str")

# Without aliases


@app.post("/optional-bytes", operation_id="optional_bytes")
async def read_optional_bytes(p: Annotated[Optional[bytes], File()] = None):
    return {"file_size": len(p) if p else None}


@app.post("/optional-uploadfile", operation_id="optional_uploadfile")
async def read_optional_uploadfile(p: Annotated[Optional[UploadFile], File()] = None):

# Without aliases


@app.post("/optional-str", operation_id="optional_str")
async def read_optional_str(p: Annotated[Optional[str], Form()] = None):
    return {"p": p}


class FormModelOptionalStr(BaseModel):
    p: Optional[str] = None


@app.post("/model-optional-str", operation_id="model_optional_str")
async def read_model_optional_str(p: Annotated[FormModelOptionalStr, Form()]):

## Usando `BackgroundTasks` { #using-backgroundtasks }

Primero, importa `BackgroundTasks` y define un parámetro en tu *path operation function* con una declaración de tipo de `BackgroundTasks`:

{* ../../docs_src/background_tasks/tutorial001_py39.py hl[1,13] *}

**FastAPI** creará el objeto de tipo `BackgroundTasks` por ti y lo pasará como ese parámetro.

# Without aliases


@app.post("/required-str", operation_id="required_str")
async def read_required_str(p: Annotated[str, Form()]):
    return {"p": p}


class FormModelRequiredStr(BaseModel):
    p: str


@app.post("/model-required-str", operation_id="model_required_str")
async def read_model_required_str(p: Annotated[FormModelRequiredStr, Form()]):

Logo, a seção com:

{% raw %}

```jinja
<a href="{{ url_for('read_item', id=id) }}">
```

{% endraw %}

...irá gerar um link para a mesma URL que será tratada pela *path operation function* `read_item(id=id)`.

Por exemplo, com um ID de `42`, isso renderizará:

```html

在文档界面中添加**路径操作**的标签和进行分组，请阅读 [教程 - 路径操作配置 - Tags 参数](../tutorial/path-operation-configuration.md#tags){.internal-link target=_blank} 文档。

## 文档的概要和描述 - OpenAPI

在文档界面中添加**路径操作**的概要和描述，请阅读 [教程 - 路径操作配置 - Summary 和 Description 参数](../tutorial/path-operation-configuration.md#summary-description){.internal-link target=_blank} 文档。

## 文档的响应描述 - OpenAPI

# Without aliases


@app.post("/list-bytes", operation_id="list_bytes")
async def read_list_bytes(p: Annotated[list[bytes], File()]):
    return {"file_size": [len(file) for file in p]}


@app.post("/list-uploadfile", operation_id="list_uploadfile")
async def read_list_uploadfile(p: Annotated[list[UploadFile], File()]):

    assert response.json()["message"] == "Successful operation."


def test_response_model_has_default_factory_return_model():
    response = client.get("/response_model_has_default_factory_return_model")

    assert response.status_code == 200, response.text

    assert response.json()["code"] == 200