# Async Tests { #async-tests }

You have already seen how to test your **FastAPI** applications using the provided `TestClient`. Up to now, you have only seen how to write synchronous tests, without using `async` functions.

OkHttp Coroutines
=================

Support for Kotlin clients using coroutines.

```kotlin
val call = client.newCall(request)

call.executeAsync().use { response ->
  withContext(Dispatchers.IO) {
    println(response.body?.string())
  }
}
```

This is implemented using `suspendCancellableCoroutine`
but uses the standard Dispatcher in OkHttp. This means
that by default Kotlin's Dispatchers are not used.

```Python hl_lines="1  4"
{!> ../../docs_src/python_types/tutorial009b_py39.py!}
```

////

#### Using `Union` or `Optional` { #using-union-or-optional }

If you are using a Python version below 3.10, here's a tip from my very **subjective** point of view:

* 🚨 Avoid using `Optional[SomeType]`
* Instead ✨ **use `Union[SomeType, None]`** ✨.

# Additional Status Codes { #additional-status-codes }

By default, **FastAPI** will return the responses using a `JSONResponse`, putting the content you return from your *path operation* inside of that `JSONResponse`.

It will use the default status code or the one you set in your *path operation*.

## Additional status codes { #additional-status-codes_1 }

   * little-endian byte order.
   *
   * @throws IOException if an I/O error occurs
   */
  @Override
  public void writeChar(int v) throws IOException {
    writeShort(v);
  }

  /**
   * Writes a {@code String} as specified by {@link DataOutputStream#writeChars(String)}, except
   * each character is written using little-endian byte order.
   *

  public static Test suite() {
    TestSuite suite = new TestSuite();

    suite.addTest(transformSuite());
    suite.addTest(filterSuite());

    suite.addTest(
        ListMultimapTestSuiteBuilder.using(
                new TestStringListMultimapGenerator() {
                  @Override
                  protected ListMultimap<String, String> create(Entry<String, String>[] entries) {

    return emptySet();
  }

  @SuppressWarnings("EmptyList") // We specifically want to test emptyList()
  public Test testsForEmptyList() {
    return ListTestSuiteBuilder.using(
            new TestStringListGenerator() {
              @Override
              public List<String> create(String[] elements) {
                return emptyList();
              }
            })

    return emptySet();
  }

  @SuppressWarnings("EmptyList") // We specifically want to test emptyList()
  public Test testsForEmptyList() {
    return ListTestSuiteBuilder.using(
            new TestStringListGenerator() {
              @Override
              public List<String> create(String[] elements) {
                return emptyList();
              }
            })

          parentBuilder) {
    List<TestSuite> derived = super.createDerivedSuites(parentBuilder);
    // TODO(cpovirk): consider using this approach (derived suites instead of extension) in
    // ListTestSuiteBuilder, etc.?
    derived.add(
        MapTestSuiteBuilder.using(new MapGenerator<K, V>(parentBuilder.getSubjectGenerator()))
            .withFeatures(parentBuilder.getFeatures())

  }

  /**
   * Searches the specified naturally ordered list for the specified object using the binary search
   * algorithm.
   *
   * <p>Equivalent to {@link #binarySearch(List, Function, Object, Comparator, KeyPresentBehavior,
   * KeyAbsentBehavior)} using {@link Ordering#natural}.
   */
  @SuppressWarnings("rawtypes") // https://github.com/google/guava/issues/989