Including external libraries also based on Pydantic, as <abbr title="Object-Relational Mapper">ORM</abbr>s, <abbr title="Object-Document Mapper">ODM</abbr>s for databases.

/// info

The parameter `response_class` will also be used to define the "media type" of the response.

In this case, the HTTP header `Content-Type` will be set to `text/html`.

And it will be documented as such in OpenAPI.

///

### Return a `Response` { #return-a-response }

## Mix `Path`, `Query` and body parameters { #mix-path-query-and-body-parameters }

First, of course, you can mix `Path`, `Query` and request body parameter declarations freely and **FastAPI** will know what to do.

And you can also declare body parameters as optional, by setting the default to `None`:

{* ../../docs_src/body_multiple_params/tutorial001_an_py310.py hl[18:20] *}

/// note

      builder.build()
    }.also { expected ->
      assertThat(expected.message).isEqualTo("Null interceptor: [null]")
    }
  }

  @Test fun nullNetworkInterceptorInList() {
    val builder = OkHttpClient.Builder()
    builder.networkInterceptors().addAll(listOf(null) as List<Interceptor>)
    assertFailsWith<IllegalStateException> {
      builder.build()
    }.also { expected ->

    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 {

 */
@Beta
public interface MutableNetwork<N, E> extends Network<N, E> {

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *
   * @return {@code true} if the network was modified as a result of this call
   */
  @CanIgnoreReturnValue
  boolean addNode(N node);

  /**

Also, before that, in previous versions, the `request` object was passed as part of the key-value pairs in the context for Jinja2.

///

/// tip

By declaring `response_class=HTMLResponse` the docs UI will be able to know that the response will be HTML.

///

/// note | Technical Details

You could also use `from starlette.templating import Jinja2Templates`.

<img src="/img/python-types/image04.png">

Now you know that you have to fix it, convert `age` to a string with `str(age)`:

{* ../../docs_src/python_types/tutorial004_py39.py hl[2] *}

## Declaring types { #declaring-types }

You just saw the main place to declare type hints. As function parameters.

   * 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}

# 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_py310.py hl[1,6:11,18:19] *}