*
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset have zero population variance
   */

   *
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset must have zero population variance
   */

   *
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset have zero population variance
   */

   *
   * <p>If the dataset contains any non-finite values ({@link Double#POSITIVE_INFINITY}, {@link
   * Double#NEGATIVE_INFINITY}, or {@link Double#NaN}) then the result is {@link
   * LinearTransformation#forNaN()}.
   *
   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset must have zero population variance
   */

Das müssen Sie selbst in Ihrem Code tun und sicherstellen, dass Sie diese JSON-Schlüssel verwenden.

Es ist fast das Einzige, woran Sie denken müssen, es selbst richtigzumachen und die Spezifikationen einzuhalten.

Den Rest erledigt **FastAPI** für Sie.

///

## Die Abhängigkeiten aktualisieren

* Hozzá tartozik kettő interaktív dokumentációs web felület.

---

Eddig csak a felszínt kapargattuk, de a lényeg hogy most már könnyebben érthető hogyan működik.

Próbáld kicserélni a következő sorban:

```Python
    return {"item_name": item.name, "item_id": item_id}
```

...ezt:

```Python
        ... "item_name": item.name ...
```

...erre:

```Python

```Python hl_lines="1  4"
{!../../docs_src/python_types/tutorial009.py!}
```

Door `Optional[str]` te gebruiken in plaats van alleen `str`, kan de editor je helpen fouten te detecteren waarbij je ervan uit zou kunnen gaan dat een waarde altijd een `str` is, terwijl het in werkelijkheid ook `None` zou kunnen zijn.

`Optional[EenType]` is eigenlijk een snelkoppeling voor `Union[EenType, None]`, ze zijn equivalent.