Let's look at how we can make that work.

## pytest.mark.anyio

If we want to call asynchronous functions in our tests, our test functions have to be asynchronous. AnyIO provides a neat plugin for this, that allows us to specify that some test functions are to be called asynchronously.

## HTTPX

Even if your **FastAPI** application uses normal `def` functions instead of `async def`, it is still an `async` application underneath.

   * calls returns to zero).
   *
   * Note: The time at which a [call][Call] is considered idle is different depending on whether it
   * was run [asynchronously][Call.enqueue] or [synchronously][Call.execute]. Asynchronous calls
   * become idle after the [onResponse][Callback.onResponse] or [onFailure][Callback.onFailure]
   * callback has returned. Synchronous calls become idle once [execute()][Call.execute] returns.

 *
 * All event methods must execute fast, without external locking, cannot throw exceptions,
 * attempt to mutate the event parameters, or be reentrant back into the client.
 * Any IO - writing to files or network should be done asynchronously.
 */
@ExperimentalOkHttpApi
abstract class ConnectionListener {
  /**
   * Invoked as soon as a call causes a connection to be started.
   */
  open fun connectStart(
    route: Route,
    call: Call,

    // Already connected. Return it immediately.
    if (plan.isReady) return ConnectResult(plan)

    // Already failed? Return it immediately.
    if (plan is FailedPlan) return plan.result

    // Connect TCP asynchronously.
    tcpConnectsInFlight += plan
    val taskName = "$okHttpName connect ${routePlanner.address.url.redact()}"
    taskRunner.newQueue().schedule(
      object : Task(taskName) {

option go_package = "k8s.io/api/certificates/v1";

// CertificateSigningRequest objects provide a mechanism to obtain x509 certificates
// by submitting a certificate signing request, and having it asynchronously approved and issued.
//
// Kubelets use this API to obtain:
//  1. client certificates to authenticate to kube-apiserver (with the "kubernetes.io/kube-apiserver-client-kubelet" signerName).

!!! tip
    Notice that here as we are using standard `open()` that doesn't support `async` and `await`, we declare the path operation with normal `def`.

### `FileResponse`

Asynchronously streams a file as the response.

Takes a different set of arguments to instantiate than the other response types:

* `path` - The filepath to the file to stream.

 *
 * All event methods must execute fast, without external locking, cannot throw exceptions, attempt
 * to mutate the event parameters, or be re-entrant back into the client. Any IO - writing to files
 * or network should be done asynchronously.
 */
abstract class EventListener {
  /**
   * Invoked as soon as a call is enqueued or executed by a client. In case of thread or stream

        .build()
    val responseB = client.newCall(requestB).execute()
    assertThat(responseB.body.string()).isEqualTo("b")
  }

  /** Make sure interceptors can interact with the OkHttp client asynchronously.  */
  @Test
  fun interceptorMakesAnUnrelatedAsyncRequest() {
    server.enqueue(MockResponse.Builder().body("a").build()) // Fetched by interceptor.

      throw InterruptedIOException()
    }
  }

  /**
   * The Okio timeout watchdog will call [timedOut] if the timeout is reached. In that case we close
   * the stream (asynchronously) which will notify the waiting thread.
   */
  internal inner class StreamTimeout : AsyncTimeout() {
    override fun timedOut() {
      closeLater(ErrorCode.CANCEL)
      connection.sendDegradedPingLater()

* **Asynchronous Code**
* **`async` and `await`**
* **Coroutines**

## Asynchronous Code

Asynchronous code just means that the language 💬 has a way to tell the computer / program 🤖 that at some point in the code, it 🤖 will have to wait for *something else* to finish somewhere else. Let's say that *something else* is called "slow-file" 📝.

So, during that time, the computer can go and do some other work, while "slow-file" 📝 finishes.