Je kunt ook een omgevingsvariabele maken die alleen voor een **specifieke programma-aanroep** beschikbaar is, die alleen voor dat programma beschikbaar is en alleen voor de duur van dat programma.

* De variabele `items_s` is een `set`, en elk van de items is van het type `bytes`.

#### Dict

Om een `dict` te definiëren, geef je 2 typeparameters door, gescheiden door komma's.

De eerste typeparameter is voor de sleutels (keys) van de `dict`.

De tweede typeparameter is voor de waarden  (values) van het `dict`:

//// tab | Python 3.9+

```Python hl_lines="1"
{!> ../../docs_src/python_types/tutorial008_py39.py!}
```

Wanneer je FastAPI installeert met `pip install "fastapi[standard]"`, worden de volgende `standard` optionele afhankelijkheden geïnstalleerd:

Gebruikt door Pydantic:

* <a href="https://github.com/JoshData/python-email-validator" target="_blank"><code>email_validator</code></a> - voor email validatie.

Gebruikt door Starlette:

* <a href="https://www.python-httpx.org" target="_blank"><code>httpx</code></a> - Vereist indien je de `TestClient` wil gebruiken.

* Allemaal **automatisch afgehandeld** door het framework.
* Alle dependencies kunnen data nodig hebben van request, de vereiste **padoperaties veranderen** en automatische documentatie verstrekken.
* **Automatische validatie** zelfs voor *padoperatie* parameters gedefinieerd in dependencies.
* Ondersteuning voor complexe gebruikersauthenticatiesystemen, **databaseverbindingen**, enz.

Your API now has the power to control its own <abbr title="This is a joke, just in case. It has nothing to do with cookie consents, but it's funny that even the API can now reject the poor cookies. Have a cookie. 🍪">cookie consent</abbr>. 🤪🍪

You can use Pydantic's model configuration to `forbid` any `extra` fields:

//// tab | Python 3.9+

```Python hl_lines="10"

   *       servers {@code alpha}, {@code bravo}, and {@code charlie} and you occasionally need to
   *       take each of the servers offline, {@code consistentHash} will be a poor fit: It provides
   *       no way for you to specify which of the three buckets is disappearing. Thus, if your
   *       buckets change from {@code [alpha, bravo, charlie]} to {@code [bravo, charlie]}, it will

     * {@link #VALIDATION_LEVEL_STRICT} should be used to ensure proper building. For the mere retrieval of dependencies
     * during artifact resolution, {@link #VALIDATION_LEVEL_MINIMAL} should be used to account for models of poor
     * quality. By default, models are validated in strict mode.
     *
     * @param validationLevel The level of validation to perform on processed models.
     * @return This request, never {@code null}.
     */

    // The Dolphin algorithm is attractive because it does the fewest array reads and writes: each
    // array slot is read and written exactly once. However, it can have very poor memory locality:
    // benchmarking shows it can take 7 times longer than the other two in some cases. The other two
    // do n swaps, minus a delta (0 or 2 for Reversal, gcd(d, n) for Successive), so that's about

   * WeakReference} (by default, strong references are used).
   *
   * <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
   * candidate for caching; consider {@link #softValues} instead.
   *
   * <p><b>Note:</b> when this method is used, the resulting cache will use identity ({@code ==})
   * comparison to determine equality of values.

   * WeakReference} (by default, strong references are used).
   *
   * <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
   * candidate for caching; consider {@link #softValues} instead.
   *
   * <p><b>Note:</b> when this method is used, the resulting cache will use identity ({@code ==})
   * comparison to determine equality of values.