And Pydantic's `Field` returns an instance of `FieldInfo` as well.

`Body` also returns objects of a subclass of `FieldInfo` directly. And there are others you will see later that are subclasses of the `Body` class.

    fun createRetry(): FakePlan {
      check(retry == null)
      return FakePlan(nextPlanId++)
        .also {
          retry = it
        }
    }

    fun createConnectTcpNextPlan(): FakePlan {
      check(connectTcpNextPlan == null)
      return FakePlan(nextPlanId++)
        .also {
          connectTcpNextPlan = it
        }
    }

    fun createConnectTlsNextPlan(): FakePlan {

The above scenario is representative of most TLS set ups: the client uses certificates to validate
the identity of a server. The converse is also possible. Here we create a server that authenticates
a client and a client that authenticates a server.

```java
// Create the root for client and server to trust. We could also use different roots for each!
HeldCertificate rootCertificate = new HeldCertificate.Builder()
    .certificateAuthority(0)

///

This is of course not the frontend for the final users, but it's a great automatic tool to document interactively all your API.

It can be used by the frontend team (that can also be yourself).

It can be used by third party applications and systems.

And it can also be used by yourself, to debug, check and test the same application.

## The `password` flow { #the-password-flow }

Now let's go back a bit and understand what is all that.

   * the discussion in the {@link #addListener} documentation. All its warnings about heavyweight
   * listeners are also applicable to heavyweight functions passed to this method.
   *
   * <p>This method is similar to {@link java.util.concurrent.CompletableFuture#exceptionally}. It
   * can also serve some of the use cases of {@link java.util.concurrent.CompletableFuture#handle}

for the Android flavor.

[Snapshot API Javadoc][guava-snapshot-api-docs] as well as
[Snapshot API Diffs][guava-snapshot-api-diffs] are also available.

Another easy way to get to the Javadoc is to open
[guava.dev/api](https://guava.dev/api). You can also jump right to a specific
class by appending the class name to guava.dev. For example,
[guava.dev/ImmutableList](https://guava.dev/ImmutableList)!

## Learn about Guava

Pass `*`, as the first parameter of the function.

Python won't do anything with that `*`, but it will know that all the following parameters should be called as keyword arguments (key-value pairs), also known as <abbr title="From: K-ey W-ord Arg-uments"><code>kwargs</code></abbr>. Even if they don't have a default value.

{* ../../docs_src/path_params_numeric_validations/tutorial003.py hl[7] *}

Let's focus on the parameters declared:

* Even though this function is a dependency ("dependable") itself, it also declares another dependency (it "depends" on something else).
    * It depends on the `query_extractor`, and assigns the value returned by it to the parameter `q`.
* It also declares an optional `last_query` cookie, as a `str`.

    * Durch die Verwendung von FastAPI sparen Sie also Entwicklungszeit, Fehler und Codezeilen und würden wahrscheinlich die gleiche Leistung (oder eine bessere) erzielen, die...

# Using Dataclasses { #using-dataclasses }

FastAPI is built on top of **Pydantic**, and I have been showing you how to use Pydantic models to declare requests and responses.

But FastAPI also supports using <a href="https://docs.python.org/3/library/dataclasses.html" class="external-link" target="_blank">`dataclasses`</a> the same way:

{* ../../docs_src/dataclasses/tutorial001.py hl[1,7:12,19:20] *}