**FastAPI** is all based on these type hints, they give it many advantages and benefits.

But even if you never use **FastAPI**, you would benefit from learning a bit about them.

/// note

If you are a Python expert, and you already know everything about type hints, skip to the next chapter.

///

## Motivation { #motivation }

Let's start with a simple example:

{* ../../docs_src/python_types/tutorial001.py *}

   * exception.
   *
   * @throws IllegalStateException when the call has already been executed.
   */
  fun enqueue(responseCallback: Callback)

  /** Cancels the request, if possible. Requests that are already complete cannot be canceled. */
  fun cancel()

  /**
   * Returns true if this call has been either [executed][execute] or [enqueued][enqueue]. It is an

   * already been determined.
   */
  public abstract <T extends @Nullable Object> ComparisonChain compare(
      @ParametricNullness T left, @ParametricNullness T right, Comparator<T> comparator);

  /**
   * Compares two {@code int} values as specified by {@link Integer#compare}, <i>if</i> the result
   * of this comparison chain has not already been determined.
   */

   * @throws IllegalStateException if a maximum size or weight was already set
   */
  @CanIgnoreReturnValue
  public CacheBuilder<K, V> maximumSize(long maximumSize) {
    checkState(
        this.maximumSize == UNSET_INT, "maximum size was already set to %s", this.maximumSize);
    checkState(
        this.maximumWeight == UNSET_INT,
        "maximum weight was already set to %s",
        this.maximumWeight);

   * uninterruptibly.
   *
   * @since 33.4.0 (but since 28.0 in the JRE flavor)
   */
  @J2ktIncompatible
  @GwtIncompatible // concurrency
  @IgnoreJRERequirement // Users will use this only if they're already using Duration.
  public static boolean awaitUninterruptibly(CountDownLatch latch, Duration timeout) {
    return awaitUninterruptibly(latch, toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
  }

  /**

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *
   * @return {@code true} if the graph was modified as a result of this call
   */
  @CanIgnoreReturnValue
  boolean addNode(N node);

  /**
   * Adds an edge connecting {@code nodeU} to {@code nodeV} if one is not already present, and sets

		} else {
			logger.Fatal(err, "Unable to initialize backend")
		}
	case errors.Is(err, errDiskFull):
		if !exit {
			storageLogOnceIf(GlobalContext, fmt.Errorf("Drive is already full at %s, incoming I/O will fail - drive will be offline", endpoint), "log-fatal-errs")
		} else {
			logger.Fatal(err, "Unable to initialize backend")
		}
	case errors.Is(err, errInconsistentDisk):
		if exit {

  /**
   * Adds {@code node} if it is not already present.
   *
   * <p><b>Nodes must be unique</b>, just as {@code Map} keys must be. They must also be non-null.
   *
   * @return {@code true} if the graph was modified as a result of this call
   */
  @CanIgnoreReturnValue
  boolean addNode(N node);

  /**
   * Adds an edge connecting {@code nodeU} to {@code nodeV} if one is not already present.
   *

          required: true
        - label: I already searched in Google "How to X in FastAPI" and didn't find any information.
          required: true
        - label: I already read and followed all the tutorial in the docs and didn't find an answer.
          required: true
        - label: I already checked if it is not related to FastAPI but to [Pydantic](https://github.com/pydantic/pydantic).

      "Cannot call source()/target() on a EndpointPair from an undirected graph. Consider calling "
          + "adjacentNode(node) if you already have a node, or nodeU()/nodeV() if you don't.";
  static final String EDGE_ALREADY_EXISTS = "Edge %s already exists in the graph.";
  static final String ENDPOINTS_MISMATCH =
      "Mismatch: endpoints' ordering is not compatible with directionality of the graph";