* The alternative documentation will also reflect the new query parameter and body:

![ReDoc](https://fastapi.tiangolo.com/img/index/index-06-redoc-02.png)

### Recap { #recap }

In summary, you declare **once** the types of parameters, body, etc. as function parameters.

You do that with standard modern Python types.

 * types, and also for interfaces and public classes that have public parameter-less constructors.
 * When the non-null default value for a particular parameter type cannot be provided by {@code
 * NullPointerTester}, the caller can provide a custom non-null default value for the parameter type
 * via {@link #setDefault}.
 *
 * @author Kevin Bourrillion
 * @since 10.0
 */
@GwtIncompatible

# Header-Parameter { #header-parameters }

Sie können Header-Parameter genauso definieren, wie Sie `Query`-, `Path`- und `Cookie`-Parameter definieren.

## `Header` importieren { #import-header }

Importieren Sie zuerst `Header`:

{* ../../docs_src/header_params/tutorial001_an_py310.py hl[3] *}

## `Header`-Parameter deklarieren { #declare-header-parameters }

 * If this parameter is not present, an empty string is returned as the content.</p>
 */
public class FilenameExtractor extends AbstractExtractor {

    /**
     * Constructs a new FilenameExtractor.
     */
    public FilenameExtractor() {
        // Default constructor
    }

    /**
     * Extracts the filename from the parameters.
     *

        }
    }

    /**
     * Retrieves an initialization parameter, converting it to the specified class type.
     * @param <T> The type of the parameter.
     * @param key The key of the parameter.
     * @param defaultValue The default value if the parameter is not found.
     * @param clazz The class type to convert the parameter to.
     * @return The parameter value, or the default value if not found.
     */

# Header Parameters { #header-parameters }

You can define Header parameters the same way you define `Query`, `Path` and `Cookie` parameters.

## Import `Header` { #import-header }

First import `Header`:

{* ../../docs_src/header_params/tutorial001_an_py310.py hl[3] *}

## Declare `Header` parameters { #declare-header-parameters }

Then declare the header parameters using the same structure as with `Path`, `Query` and `Cookie`.

        </parameter>
        <parameter>
          <name>repositories</name>
          <type>java.util.List</type>
          <required>true</required>
          <editable>false</editable>
        </parameter>
        <parameter>
          <name>resourceBundles</name>
          <type>java.util.List</type>
          <required>true</required>
          <editable>true</editable>
        </parameter>
        <parameter>

_Unset: Any = Undefined


def Path(  # noqa: N802
    default: Annotated[
        Any,
        Doc(
            """
            Default value if the parameter field is not set.

            This doesn't affect `Path` parameters as the value is always required.
            The parameter is available only for compatibility.
            """
        ),
    ] = ...,
    *,
    default_factory: Annotated[

If you pass a "callable" as a dependency in **FastAPI**, it will analyze the parameters for that "callable", and process them in the same way as the parameters for a *path operation function*. Including sub-dependencies.

That also applies to callables with no parameters at all. The same as it would be for *path operation functions* with no parameters.

        return builder.toString();
    }

    @Override
    public void visitTypes(Action<TypeMetaData> action) {
        action.execute(returnType);
        for (ParameterMetaData parameter : parameters) {
            parameter.visitTypes(action);
        }
    }