* LF<? extends @Nullable V>. That might be better: There's currently no difference between the
     * outputs users get when calling this with <Foo> and calling it with <@Nullable Foo>. The only
     * difference is that calling it with <Foo> won't work when an input Future has a @Nullable
     * type. So why even make that error possible by giving callers the choice?
     *

    FinalizableReference.register(this);
  }

  /**
   * Returns a future that finishes when this step does. Calling {@code get()} on the returned
   * future returns {@code null} if the step is successful or throws the same exception that would
   * be thrown by calling {@code finishToFuture().get()} if this were the last step. Calling {@code
   * cancel()} on the returned future has no effect on the {@code ClosingFuture} pipeline.
   *

But **FastAPI** will know that it has to solve `query_extractor` first, to pass the results of that to `query_or_cookie_extractor` while calling it.

///

```mermaid
graph TB

query_extractor(["query_extractor"])
query_or_cookie_extractor(["query_or_cookie_extractor"])

read_query["/items/"]

/// tip

You'll see what other "things", apart from functions, can be used as dependencies in the next chapter.

///

Whenever a new request arrives, **FastAPI** will take care of:

* Calling your dependency ("dependable") function with the correct parameters.
* Get the result from your function.
* Assign that result to the parameter in your *path operation function*.

```mermaid
graph TB

   * note that the execution order of all listeners is ultimately chosen by the implementations of
   * the supplied executors.
   *
   * <p>This method is idempotent. Calling it several times in parallel is semantically equivalent
   * to calling it exactly once.
   *
   * @since 10.0 (present in 1.0 as {@code run})
   */
  public void execute() {

{* ../../docs_src/encoder/tutorial001_py310.py hl[4,21] *}

In this example, it would convert the Pydantic model to a `dict`, and the `datetime` to a `str`.

The result of calling it is something that can be encoded with the Python standard [`json.dumps()`](https://docs.python.org/3/library/json.html#json.dumps).

   * #wasInterrupted} as necessary. This ensures that the work is done even if the future is
   * cancelled without a call to {@code cancel}, such as by calling {@code
   * setFuture(cancelledFuture)}.
   *
   * <p>Beware of completing a future while holding a lock. Its listeners may do slow work or
   * acquire other locks, risking deadlocks.
   */
  @CanIgnoreReturnValue

  }

  public void testAddAfterRun() throws Exception {
    // Run the previous test
    testRunOnPopulatedList();

    // If it passed, then verify an Add will be executed without calling run
    CountDownLatch countDownLatch = new CountDownLatch(1);
    list.add(new MockRunnable(countDownLatch), newCachedThreadPool());
    assertTrue(countDownLatch.await(1L, SECONDS));
  }

  }

  public void testAddAfterRun() throws Exception {
    // Run the previous test
    testRunOnPopulatedList();

    // If it passed, then verify an Add will be executed without calling run
    CountDownLatch countDownLatch = new CountDownLatch(1);
    list.add(new MockRunnable(countDownLatch), newCachedThreadPool());
    assertTrue(countDownLatch.await(1L, SECONDS));
  }

    final ImmutableList<Timeout> timeouts;

    TimeoutsToUse(Timeout... timeouts) {
      this.timeouts = ImmutableList.copyOf(timeouts);
    }
  }

  /** Possible outcomes of calling any of the methods under test. */
  private enum Outcome {

    /** The method returned normally and is either void or returned true. */
    SUCCESS,

    /** The method returned false. */
    FAILURE,