Pydantic heeft een speciaal gedrag wanneer je `Optional` of `Union[EenType, None]` gebruikt zonder een standaardwaarde, je kunt er meer over lezen in de Pydantic-documentatie over <a href="https://docs.pydantic.dev/2.3/usage/models/#required-fields" class="external-link" target="_blank">Verplichte optionele velden</a>.

///

## Type Hints met Metadata Annotaties

///

### Использование Pydantic's `Required` вместо Ellipsis (`...`)

    val point = Point(null, 9L)
    assertThat(Point.ADAPTER.fromDer(bytes)).isEqualTo(point)
    assertThat(Point.ADAPTER.toDer(point)).isEqualTo(bytes)
  }

  @Test fun `point with both fields set`() {
    val bytes = "3006800109810109".decodeHex()
    val point = Point(9L, 9L)
    assertThat(Point.ADAPTER.fromDer(bytes)).isEqualTo(point)
    assertThat(Point.ADAPTER.toDer(point)).isEqualTo(bytes)
  }

    public static final String ALL_LANGUAGES = "all";

    public static final String INVALID_NUMERIC_PARAMETER = "-1";

    public static final String FACET_FIELD_PREFIX = "field:";

    public static final String FACET_QUERY_PREFIX = "query:";

    public static final String MATCHES_ALL_QUERY = "*:*";

    @Deprecated

  }

  final transient ImmutableMap<K, ? extends ImmutableCollection<V>> map;
  final transient int size;

  // These constants allow the deserialization code to set final fields. This
  // holder class makes sure they are not initialized unless an instance is
  // deserialized.
  @GwtIncompatible // java serialization is not supported
  @J2ktIncompatible
  static class FieldSettersHolder {

type validator struct{}

func checkFields(un *unstructured.Unstructured) error {
	var errs error
	for key := range un.Object {
		if _, ok := validFields[key]; !ok {
			errs = multierror.Append(errs, fmt.Errorf("unknown field %q", key))
		}
	}
	return errs
}

func (v *validator) validateResource(istioNamespace, defaultNamespace string, un *unstructured.Unstructured, writer io.Writer) (validation.Warning, error) {

	if errCode != ErrUnsignedHeaders {
		t.Fatalf("Expected the APIErrorCode to %d, but got %d", ErrUnsignedHeaders, errCode)
	}

	// case where the list of signed headers doesn't contain the host field.
	signedHeaders = signedHeaders[2:5]
	// expected to fail with `ErrUnsignedHeaders`.
	_, errCode = extractSignedHeaders(signedHeaders, r)
	if errCode != ErrUnsignedHeaders {

You can use all the same validation features and tools you use for Pydantic models, like different data types and additional validations with `Field()`.

//// tab | Pydantic v2

```Python hl_lines="2  5-8  11"
{!> ../../docs_src/settings/tutorial001.py!}
```

////

//// tab | Pydantic v1

/// info

🗄 `BaseSettings` ⚪️➡️ Pydantic & ✍ 🎧-🎓, 📶 🌅 💖 ⏮️ Pydantic 🏷.

🎏 🌌 ⏮️ Pydantic 🏷, 👆 📣 🎓 🔢 ⏮️ 🆎 ✍, & 🎲 🔢 💲.

👆 💪 ⚙️ 🌐 🎏 🔬 ⚒ & 🧰 👆 ⚙️ Pydantic 🏷, 💖 🎏 📊 🆎 & 🌖 🔬 ⏮️ `Field()`.

```Python hl_lines="2  5-8  11"
{!../../docs_src/settings/tutorial001.py!}
```

/// tip

🚥 👆 💚 🕳 ⏩ 📁 & 📋, 🚫 ⚙️ 👉 🖼, ⚙️ 🏁 1️⃣ 🔛.

///

  /** The parts of the domain name, converted to lower case. */
  private final ImmutableList<String> parts;

  /**
   * Cached value of #publicSuffixIndex(). Do not use directly.
   *
   * <p>Since this field isn't {@code volatile}, if an instance of this class is shared across
   * threads before it is initialized, then each thread is likely to compute their own copy of the
   * value.
   */
  @SuppressWarnings("Immutable")