* could use null, but that messes with our nullness checking, including under J2KT. We could
   * probably work around it by changing how we annotate the J2CL EnumMap, but that's probably more
   * trouble than just using Object.class.)
   *
   * Then we declare the getters for these fields as @GwtIncompatible so that no one can try to use
   * them under J2CL—or, as an unfortunate side effect, under GWT. We do still give the fields

<img src="/img/tutorial/separate-openapi-schemas/image02.png">
</div>

This means that it will **always have a value**, it's just that sometimes the value could be `None` (or `null` in JSON).

That means that, clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.

# Test Client - `TestClient`

You can use the `TestClient` class to test FastAPI applications without creating an actual HTTP and socket connection, just communicating directly with the FastAPI code.

Read more about it in the [FastAPI docs for Testing](https://fastapi.tiangolo.com/tutorial/testing/).

You can import it directly from `fastapi.testclient`:

```python
from fastapi.testclient import TestClient
```

For example, if you want to read or manipulate the request body before it is processed by your application.

/// danger

This is an "advanced" feature.

If you are just starting with **FastAPI** you might want to skip this section.

///

## Use cases { #use-cases }

Some use cases include:

    try {
      long remainingNanos = unit.toNanos(timeout);
      long end = System.nanoTime() + remainingNanos;

      while (true) {
        try {
          // CountDownLatch treats negative timeouts just like zero.
          return latch.await(remainingNanos, NANOSECONDS);
        } catch (InterruptedException e) {
          interrupted = true;
          remainingNanos = end - System.nanoTime();
        }
      }

/// tip

The `model_config` attribute is used just for Pydantic configuration. You can read more at <a href="https://docs.pydantic.dev/latest/concepts/config/" class="external-link" target="_blank">Pydantic: Concepts: Configuration</a>.

///

////

//// tab | Pydantic v1

{* ../../docs_src/settings/app03_an/config_pv1.py hl[9:10] *}

/// tip

    // These contain bytes not in the decodabet.
    assertFailsToDecode(base64(), "A\u007f", "Unrecognized character: 0x7f");
    assertFailsToDecode(base64(), "Wf2!", "Unrecognized character: !");
    // This sentence just isn't base64() encoded.
    assertFailsToDecode(base64(), "let's not talk of love or chains!");
    // A 4n+1 length string is never legal base64().
    assertFailsToDecode(base64(), "12345", "Invalid input length 5");

        assertEquals(6, NbtAddress.UNKNOWN_MAC_ADDRESS.length);
    }

    @Test
    void testConstantsAreImmutable() {
        // Ensure constants are properly defined as final
        // This test just verifies the constants exist and are accessible
        String anyHosts = NbtAddress.ANY_HOSTS_NAME;
        String masterBrowser = NbtAddress.MASTER_BROWSER_NAME;
        String smbServer = NbtAddress.SMBSERVER_NAME;

  @Test
  fun testToStringIsClassname() {
    assertThat(Platform().toString()).isEqualTo("Platform")
  }

  @Test
  fun testNotAndroid() {
    platform.assumeNotAndroid()

    // This is tautological so just confirms that it runs.
    assertThat(isAndroid).isEqualTo(false)
  }

        #### Utility: compare with alternatives


        There is always *some* alternative to adding a new feature to Guava, even if it's just
        forking Guava yourself.


        We want to see that new features have some significant advantage over the alternatives.
        These advantages can take