# Body - Nested Models { #body-nested-models }

With **FastAPI**, you can define, validate, document, and use arbitrarily deeply nested models (thanks to Pydantic).

## List fields { #list-fields }

You can define an attribute to be a subtype. For example, a Python `list`:

{* ../../docs_src/body_nested_models/tutorial001_py310.py hl[12] *}

This will make `tags` be a list, although it doesn't declare the type of the elements of the list.

/// warning | Uyarı

`Field`'a geçirilen ekstra key'ler, uygulamanız için üretilen OpenAPI schema'sında da yer alır.
Bu key'ler OpenAPI spesifikasyonunun bir parçası olmak zorunda olmadığından, örneğin [OpenAPI validator](https://validator.swagger.io/) gibi bazı OpenAPI araçları üretilen schema'nızla çalışmayabilir.

///

## Özet { #recap }

}
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
$webclient.DownloadFile($distributionUrl, "$TMP_DOWNLOAD_DIR/$distributionUrlName") | Out-Null

# If specified, validate the SHA-256 sum of the Maven distribution zip file
$distributionSha256Sum = (Get-Content -Raw "$scriptDir/.mvn/wrapper/maven-wrapper.properties" | ConvertFrom-StringData).distributionSha256Sum
if ($distributionSha256Sum) {

app = FastAPI()

@app.get("/items/invalidnone", response_model=Union[Item, None])
def get_invalid_none():
    return None
```

This way the data will be correctly validated, you won't have an internal server error, and the documentation will also reflect that this *path operation* could return `None` (or `null` in JSON).

### Fixes

import org.junit.jupiter.api.Test;

/**
 * Security-focused test cases for BufferCache to verify buffer overflow protection.
 */
public class BufferCacheSecurityTest {

    /**
     * Test that buffer allocation validates size to prevent overflow.
     */
    @Test
    public void testBufferSizeValidation() {
        // When - Get a buffer
        byte[] buffer = BufferCache.getBuffer();

     * @return HTML response for the web config edit form
     */
    @Execute
    @Secured({ ROLE })
    public HtmlResponse edit(final EditForm form) {
        validate(form, messages -> {}, this::asListHtml);
        final String id = form.id;
        webConfigService.getWebConfig(id).ifPresent(entity -> {
            copyBeanToBean(entity, form, copyOp -> {

import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import java.util.stream.StreamSupport;

/**
 * Incremental task to validate that the API names in set of JSON files do not contain
 * programming language keywords.
 * <p>
 * The keywords are defined in a JSON file, although it's worth noting that what is and isn't a

		fmt.Fprintln(os.Stderr, "reading input:", err)
	}
	fmt.Printf("%d\n", count)
	// Output: 15
}

// Use a Scanner with a custom split function (built by wrapping ScanWords) to validate
// 32-bit decimal input.
func ExampleScanner_custom() {
	// An artificial input source.
	const input = "1234 5678 1234567901234567890"
	scanner := bufio.NewScanner(strings.NewReader(input))

	}
}

// RetentionDate is a embedded type containing time.Time to unmarshal
// Date in Retention
type RetentionDate struct {
	time.Time
}

// UnmarshalXML parses date from Retention and validates date format
func (rDate *RetentionDate) UnmarshalXML(d *xml.Decoder, startElement xml.StartElement) error {
	var dateStr string
	err := d.DecodeElement(&dateStr, &startElement)
	if err != nil {
		return err
	}

        return determineValid(value);
    }

    /**
     * Determines if the given value is a valid cron expression.
     * @param value the value to validate
     * @return true if valid, false otherwise
     */
    protected boolean determineValid(final String value) {
        if (StringUtil.isNotBlank(value) && !LaCronUtil.isCronExpValid(value)) {
            return false;