```

### More technical details { #more-technical-details }

You might have noticed that `await` can only be used inside of functions defined with `async def`.

But at the same time, functions defined with `async def` have to be "awaited". So, functions with `async def` can only be called inside of functions defined with `async def` too.

So, about the egg and the chicken, how do you call the first `async` function?

{* ../../docs_src/header_param_models/tutorial001_an_py310.py hl[9:14,18] *}

**FastAPI** will **extract** the data for **each field** from the **headers** in the request and give you the Pydantic model you defined.

## Check the Docs { #check-the-docs }

You can see the required headers in the docs UI at `/docs`:

<div class="screenshot">
<img src="/img/tutorial/header-param-models/image01.png">
</div>

type Source struct {
	Host      string `json:"host"`
	Port      string `json:"port"`
	UserAgent string `json:"userAgent"`
}

// Event represents event notification information defined in
// http://docs.aws.amazon.com/AmazonS3/latest/dev/notification-content-structure.html.
type Event struct {
	EventVersion      string            `json:"eventVersion"`
	EventSource       string            `json:"eventSource"`

///

### Pydantic v1 and v2 on the same app { #pydantic-v1-and-v2-on-the-same-app }

It's **not supported** by Pydantic to have a model of Pydantic v2 with its own fields defined as Pydantic v1 models or vice versa.

```mermaid
graph TB
    subgraph "❌ Not Supported"
        direction TB
        subgraph V2["Pydantic v2 Model"]
            V1Field["Pydantic v1 Model"]
        end

 *
 * @since 4.0.0
 */
@Experimental
@Immutable
public interface Lifecycle extends ExtensibleEnum {

    // =========================
    // Maven defined lifecycles
    // =========================
    String CLEAN = "clean";
    String DEFAULT = "default";
    String SITE = "site";

    // ======================
    // Phase qualifiers

     * and model completely decoupled by injecting deep copies rather than the original objects from the profile.
     *
     * @param model The model into which to merge the values defined by the profile, must not be <code>null</code>.
     * @param profile The (read-only) profile whose values should be injected, may be <code>null</code>.

		}
		names = append(names, c.name)
	}

	sortCallback = func(c *callback) error {
		if c.before != "" { // if defined before callback
			if c.before == "*" && len(sorted) > 0 {
				if curIdx := getRIndex(sorted, c.name); curIdx == -1 {
					sorted = append([]string{c.name}, sorted...)
				}

 * environment. This interface allows users to define and configure
 * Java-related toolchains that can be utilized during the build process
 * in Maven.</p>
 *
 * <p>Java toolchains are defined in the Maven toolchains.xml file and can
 * be referenced in the project's POM file. This enables developers to
 * specify the exact versions of Java tools they wish to use, ensuring

import org.apache.maven.artifact.versioning.OverConstrainedVersionException;

/**
 * Resolves conflicting artifacts by always selecting the newest declaration. Newest is defined as the
 * declaration whose version is greater according to <code>ArtifactVersion.compareTo</code>.
 *
 * @see ArtifactVersion#compareTo
 * @since 3.0
 */
@Named("newest")
@Singleton
@Deprecated

    """

    def __init__(
        self,
        code: Annotated[
            int,
            Doc(
                """
                A closing code from the
                [valid codes defined in the specification](https://datatracker.ietf.org/doc/html/rfc6455#section-7.4.1).
                """
            ),
        ],
        reason: Annotated[
            Union[str, None],
            Doc(