}

    /**
     * Stores a parameter value with the specified key.
     *
     * @param key the parameter key, must not be null
     * @param value the parameter value, may be null
     */
    public void put(final String key, final Object value) {
        params.put(key, value);
    }

    /**
     * Retrieves a parameter value by key.
     *
     * @param key the parameter key to look up

# Pfad-Parameter { #path-parameters }

Sie können Pfad-„Parameter“ oder -„Variablen“ mit der gleichen Syntax deklarieren, welche in Python-<abbr title="Formatstring – Formatierter String: Der String enthält Ausdrücke, die mit geschweiften Klammern umschlossen sind. Solche Stellen werden durch den Wert des Ausdrucks ersetzt">Formatstrings</abbr> verwendet wird:

{* ../../docs_src/path_params/tutorial001_py310.py hl[6:7] *}

In this case, the function parameter `q` will be optional, and will be `None` by default.

/// check

Also notice that **FastAPI** is smart enough to notice that the path parameter `item_id` is a path parameter and `q` is not, so, it's a query parameter.

///

## Query parameter type conversion { #query-parameter-type-conversion }

You can also declare `bool` types, and they will be converted:

# Paramètres d'en-tête { #header-parameters }

Vous pouvez définir des paramètres `Header` de la même manière que vous définissez des paramètres `Query`, `Path` et `Cookie`.

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

Commencez par importer `Header` :

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

## Déclarer des paramètres `Header` { #declare-header-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.
     */

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.

# 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 }

    // for error page

    /**
     * Query string parameter for error page fallback.
     * Contains the original search query if redirection fails.
     */
    public String q;

    /**
     * Number of results parameter for error page fallback.
     * Specifies how many results to display if redirection fails.
     */
    public String num;

    /**
     * Sort parameter for error page fallback.

    /**
     * Add the created action message for the key 'constraints.Size.message' with parameters.
     * <pre>
     * message: The size of {item} must be between {min} and {max}.
     * </pre>
     * @param property The property name for the message. (NotNull)
     * @param min The parameter min for message. (NotNull)
     * @param max The parameter max for message. (NotNull)
     * @return this. (NotNull)
     */

# Header Parameter Models { #header-parameter-models }

If you have a group of related **header parameters**, you can create a **Pydantic model** to declare them.

This would allow you to **re-use the model** in **multiple places** and also to declare validations and metadata for all the parameters at once. 😎

/// note

This is supported since FastAPI version `0.115.0`. 🤓

///