...then because `description` has a default value, if you **don't return anything** for that field, it will still have that **default value**.

### Model for Output Response Data { #model-for-output-response-data }

If you interact with the docs and check the response, even though the code didn't add anything in one of the `description` fields, the JSON response contains the default value (`null`):

<div class="screenshot">

<div class="screenshot">
<img src="/img/tutorial/request-form-models/image01.png">
</div>

## Forbid Extra Form Fields { #forbid-extra-form-fields }

In some special use cases (probably not very common), you might want to **restrict** the form fields to only those declared in the Pydantic model. And **forbid** any **extra** fields.

/// note

This is supported since FastAPI version `0.114.0`. 🤓

///

- **Security:**
Java serialization poses security risks, especially related to deserialization vulnerabilities.

- **Version Compatibility:**
With Java serialization, even minor changes to a class (like adding a field) can break compatibility.

- **Cross-Language Compatibility:**

In some special use cases (probably not very common), you might want to **restrict** the headers that you want to receive.

You can use Pydantic's model configuration to `forbid` any `extra` fields:

{* ../../docs_src/header_param_models/tutorial002_an_py310.py hl[10] *}

If a client tries to send some **extra headers**, they will receive an **error** response.

        </field>
        <field>
          <name>exists</name>
          <version>1.0.0+</version>
          <type>String</type>
          <description>
            The name of the file that should exist to activate a profile. Please note, that missing and exists
            fields cannot be used together. Only one of them should be used at any one time.
          </description>
        </field>
      </fields>
    </class>

class Session:
    def __init__(self) -> None:
        self.open = True


def dep_session() -> Any:
    s = Session()
    yield s
    s.open = False


def raise_after_yield() -> Any:
    yield
    raise HTTPException(status_code=503, detail="Exception after yield")


SessionFuncDep = Annotated[Session, Depends(dep_session, scope="function")]

				if idx.Option == "" {
					idx.Option = index.Option
				}

				idx.Fields = append(idx.Fields, index.Fields...)
				sort.Slice(idx.Fields, func(i, j int) bool {
					return idx.Fields[i].Priority < idx.Fields[j].Priority
				})
			}
		}
	}
	for _, index := range indexes {
		if index.Class == "UNIQUE" && len(index.Fields) == 1 {
			index.Fields[0].Field.UniqueIndex = index.Name
		}
	}
	return indexes
}

    - tutorial/body.md
    - tutorial/query-params-str-validations.md
    - tutorial/path-params-numeric-validations.md
    - tutorial/query-param-models.md
    - tutorial/body-multiple-params.md
    - tutorial/body-fields.md
    - tutorial/body-nested-models.md
    - tutorial/schema-extra-example.md
    - tutorial/extra-data-types.md
    - tutorial/cookie-params.md
    - tutorial/header-params.md
    - tutorial/cookie-param-models.md

Puedes leer más al respecto en la documentación de [Dependencias con `yield` - Salida temprana y `scope`](../tutorial/dependencies/dependencies-with-yield.md#early-exit-and-scope).

### Dependencias con `yield` y `StreamingResponse`, detalles técnicos { #dependencies-with-yield-and-streamingresponse-technical-details }

     * we can reference a node whose fields have been cleared is inside the iterator (and presumably
     * only under concurrent modification).
     *
     * To access these fields when you know that they are not null, call the pred() and succ()
     * methods, which perform null checks before returning the fields.
     */
    private @Nullable AvlNode<E> pred;