*
     * @return the message builder factory
     */
    @Nonnull
    MessageBuilderFactory messageBuilderFactory();

    /**
     * Returns the command-line arguments to be parsed.
     *
     * @return a list of argument strings
     */
    @Nonnull
    List<String> args();

    /**
     * Per-request {@link Lookup} for customization.
     *
     * @return a lookup possibly with custom components

///

The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback { #documenting-the-callback }

The actual callback code will depend heavily on your own API app.

	// There are two possibilities:
	// - We use a KMS -> There must be non-empty key ID and a KMS data key.
	// - We use a K/V -> There must be no key ID and no KMS data key.
	// Otherwise, the caller has passed an invalid argument combination.
	if keyID == "" && len(kmsKey) != 0 {
		logger.CriticalIf(context.Background(), errors.New("The key ID must not be empty if a KMS data key is present"))
	}
	if keyID != "" && len(kmsKey) == 0 {

///

Now run the live server for the docs in Spanish:

<div class="termy">

```console
// Use the command "live" and pass the language code as a CLI argument
$ python ./scripts/docs.py live es

<span style="color: green;">[INFO]</span> Serving on http://127.0.0.1:8008
<span style="color: green;">[INFO]</span> Start watching changes

    // parseable, but the spec doesn't seem to say which exception is thrown for an invalid radix.
    // In contrast to the JVM, Kotlin native throws an Illegal argument exception in this case
    // (which seems to make more sense).
    try {
      UnsignedInts.parseUnsignedInt("0", Character.MIN_RADIX - 1);
      fail();
    } catch (NumberFormatException expected) {

///

Das einzig Neue ist `callbacks=invoices_callback_router.routes` als Argument für den *Pfadoperation-Dekorator*. Wir werden als Nächstes sehen, was das ist.

## Dokumentation des Callbacks { #documenting-the-callback }

Der tatsächliche Callback-Code hängt stark von Ihrer eigenen API-Anwendung ab.

For example you could have a file `main.py` with:

```Python hl_lines="3"
import os

name = os.getenv("MY_NAME", "World")
print(f"Hello {name} from Python")
```

/// tip

The second argument to <a href="https://docs.python.org/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> is the default value to return.

            try {
                if (input.args().length == 1) {
                    shellContext.cwd.change(input.args()[0]);
                } else {
                    shellContext.logger.error("Command accepts only one argument");
                }
            } catch (Exception e) {
                saveException(e);
            }
        }

        private List<Completer> cdCompleter(String name) {

{* ../../docs_src/path_params/tutorial001_py39.py hl[6:7] *}

Der Wert des Pfad-Parameters `item_id` wird Ihrer Funktion als das Argument `item_id` übergeben.

   * by the argument according to three criteria:
   *
   * <ol>
   *   <li>The type of the argument is the wildcard or equal to the type of this instance.
   *   <li>The subtype of the argument is the wildcard or equal to the subtype of this instance.
   *   <li>All of the parameters present in the argument are present in this instance.
   * </ol>
   *
   * <p>For example: