But we want to be able to parameterize that fixed content.

## A "callable" instance { #a-callable-instance }

In Python there's a way to make an instance of a class a "callable".

Not the class itself (which is already a callable), but an instance of that class.

To do that, we declare a method `__call__`:

{* ../../docs_src/dependencies/tutorial011_an_py39.py hl[12] *}

    Callable<@Nullable Void> callable =
        new Callable<@Nullable Void>() {
          @Override
          public @Nullable Void call() throws Exception {
            assertEquals(Thread.currentThread().getName(), newName.get());
            return null;
          }
        };
    Callables.threadRenaming(callable, newName).call();

   * <p>Cancellation does not propagate from the output future to a callable that has begun to
   * execute, but if the output future is cancelled before {@link Callable#call()} is invoked,
   * {@link Callable#call()} will not be invoked.
   */
  public <T extends @Nullable Object> ListenableFuture<T> submit(
      Callable<T> callable, Executor executor) {
    checkNotNull(callable);
    checkNotNull(executor);
    return submitAsync(

    // HcHttpClient.AUTHENTICATIONS_PROPERTY,
    // basicAuthList.toArray(new Authentication[basicAuthList.size()]));
    //
    // List<Callable<ResponseData>> list =
    // new ArrayList<Callable<ResponseData>>();
    // for (int i = 0; i < 100; i++) {
    // list.add(new Callable<ResponseData>() {
    // public ResponseData call() throws Exception {
    // String[] urls =
    // new String[] {
    // "http://.../",

          }
        };

    results.add(serializer.submit(Callables.returning(null), manualExecutor));
    Future<?>[] thingToCancel = new Future<?>[1];
    results.add(
        serializer.submit(
            new Callable<@Nullable Void>() {
              @Override
              public @Nullable Void call() {
                thingToCancel[0].cancel(false);
                return null;

    runConcurrentTest(
        numberOfThreads,
        new Callable<@Nullable Void>() {
          @Override
          public @Nullable Void call() throws Exception {
            long unused = ticker.read();
            return null;
          }
        });

    assertEquals(incrementByNanos * numberOfThreads, ticker.read());
  }

  /** Runs {@code callable} concurrently {@code numberOfThreads} times. */

    return schedule(callable(command), delay, unit);
  }

  @Override
  public <V> ListenableScheduledFuture<V> schedule(
      Callable<V> callable, long delay, TimeUnit unit) {
    Preconditions.checkNotNull(callable, "callable must not be null!");
    Preconditions.checkNotNull(unit, "unit must not be null!");
    ListenableFuture<V> delegateFuture = submit(callable);

    return schedule(callable(command), delay, unit);
  }

  @Override
  public <V> ListenableScheduledFuture<V> schedule(
      Callable<V> callable, long delay, TimeUnit unit) {
    Preconditions.checkNotNull(callable, "callable must not be null!");
    Preconditions.checkNotNull(unit, "unit must not be null!");
    ListenableFuture<V> delegateFuture = submit(callable);

    CountDownLatch enterLatch = new CountDownLatch(1);
    CountDownLatch exitLatch = new CountDownLatch(1);
    TrustedListenableFutureTask<@Nullable Void> task =
        TrustedListenableFutureTask.create(
            new Callable<@Nullable Void>() {
              @Override
              public @Nullable Void call() throws Exception {
                enterLatch.countDown();
                new CountDownLatch(1).await(); // wait forever

So, a Python class is also a **callable**.

Then, in **FastAPI**, you could use a Python class as a dependency.

What FastAPI actually checks is that it is a "callable" (function, class or anything else) and the parameters defined.