/// tip

You can set a dependency override for a dependency used anywhere in your **FastAPI** application.

The original dependency could be used in a *path operation function*, a *path operation decorator* (when you don't use the return value), a `.include_router()` call, etc.

FastAPI will still be able to override it.

///

Let's imagine you want to get the client's IP address/host inside of your *path operation function*.

For that you need to access the request directly.

{* ../../docs_src/using_request_directly/tutorial001_py39.py hl[1,7:8] *}

By declaring a *path operation function* parameter with the type being the `Request` **FastAPI** will know to pass the `Request` in that parameter.

/// tip

Similarly, you cannot redefine a path operation:

{* ../../docs_src/path_params/tutorial003b_py39.py hl[6,11] *}

The first one will always be used since the path matches first.

## Predefined values { #predefined-values }

De manera similar, no puedes redefinir una path operation:

{* ../../docs_src/path_params/tutorial003b_py39.py hl[6,11] *}

La primera siempre será utilizada ya que el path coincide primero.

## Valores predefinidos { #predefined-values }

///

Este parámetro no crea ese endpoint / *path operation*, pero declara que la URL `/token` será la que el cliente deberá usar para obtener el token. Esa información se usa en OpenAPI, y luego en los sistemas de documentación interactiva del API.

Pronto también crearemos la verdadera *path operation*.

/// info | Información

     * Calculates a MAC using the ARCFOUR-HMAC-MD5 algorithm.
     * This method implements the Microsoft variant of the Kerberos ARCFOUR-HMAC-MD5 checksum.
     *
     * @param keyusage the Kerberos key usage number for this operation
     * @param key the encryption key to use for MAC calculation
     * @param data the data to calculate the MAC for
     * @return the calculated MAC bytes

import (
	"encoding/binary"
	"fmt"
	"strings"
	"time"

	"github.com/tinylib/msgp/msgp"
	"github.com/zeebo/xxh3"
)

// Op is operation type.
//
//go:generate msgp -unexported -file=$GOFILE
//go:generate stringer -type=Op -output=msg_string.go -trimprefix=Op $GOFILE

// Op is operation type messages.
type Op uint8

// HandlerID is the ID for the handler of a specific type.
type HandlerID uint8

const (

# Background Tasks - `BackgroundTasks`

You can declare a parameter in a *path operation function* or dependency function with the type `BackgroundTasks`, and then you can use it to schedule the execution of background tasks after the response is sent.

You can import it directly from `fastapi`:

```python
from fastapi import BackgroundTasks
```

///

Les réponses générées au format OpenAPI pour cette *opération de chemin* seront :

```JSON hl_lines="3-12"
{
    "responses": {
        "404": {
            "description": "Additional Response",
            "content": {
                "application/json": {

            // Now perform a write operation
            wrapperProps.setProperty("new.key", "new.value");

            // Read the initial key again - should return the current value
            assertEquals("externally.modified", wrapperProps.getProperty("initial.key"));

            // Read the external key that was set before the write operation