The benefit of raising an exception over returning a value will be more evident in the section about Dependencies and Security.

In this example, when the client requests an item by an ID that doesn't exist, raise an exception with a status code of `404`:

{* ../../docs_src/handling_errors/tutorial001.py hl[11] *}

///

/// tip

If you need to do any type of validation that requires communicating with any **external component**, like a database or another API, you should instead use **FastAPI Dependencies**, you will learn about them later.

These custom validators are for things that can be checked with **only** the **same data** provided in the request.

///

### Understand that Code { #understand-that-code }

### Wstrzykiwanie Zależności

FastAPI zawiera niezwykle łatwy w użyciu, ale niezwykle potężny system <abbr title='Po angielsku: Dependency Injection. Znane również jako "components", "resources", "services", "providers"'><strong>Wstrzykiwania Zależności</strong></abbr>.

* Nawet zależności mogą mieć zależności, tworząc hierarchię lub **"graf" zależności**.

                String value = get(propertyKey);
                return "true".equalsIgnoreCase(value);
            }

            // Override getDayForCleanup to avoid ComponentUtil dependency
            public int getDayForCleanup() {
                return 3;
            }
        };
        ComponentUtil.register(fessConfig, FessConfig.class.getCanonicalName());
    }

## External Dependency Version Information

Continuous integration builds have used the following versions of external dependencies, however, this is not a strong recommendation and users should consult an appropriate installation or upgrade guide before deciding what versions of etcd, docker or rkt to use.

    cache.put(1, 10);
    cache.put(2, 20);
    cache.put(3, 30);
    cache.put(4, 40);
    cache.put(5, 50);

    assertEquals(null, cache.getIfPresent(10));
    // Order required to remove dependence on access order / write order constraint.
    assertEquals(Integer.valueOf(20), cache.getIfPresent(2));
    assertEquals(Integer.valueOf(30), cache.getIfPresent(3));
    assertEquals(Integer.valueOf(40), cache.getIfPresent(4));

 *       APIs are offered that don't).
 *   <li>Can't be passed directly to methods that expect {@code long[]} (though the most common
 *       utilities do have replacements here).
 *   <li>Dependency on {@code com.google.common} / Guava.
 * </ul>
 *
 * <p>Advantages compared to {@link com.google.common.collect.ImmutableList ImmutableList}{@code
 * <Long>}:
 *
 * <ul>

 *       APIs are offered that don't).
 *   <li>Can't be passed directly to methods that expect {@code int[]} (though the most common
 *       utilities do have replacements here).
 *   <li>Dependency on {@code com.google.common} / Guava.
 * </ul>
 *
 * <p>Advantages compared to {@link com.google.common.collect.ImmutableList ImmutableList}{@code
 * <Integer>}:
 *
 * <ul>

 *
 * <p><b>We recommend against using EventBus.</b> It was designed many years ago, and newer
 * libraries offer better ways to decouple components and react to events.
 *
 * <p>To decouple components, we recommend a dependency-injection framework. For Android code, most
 * apps use <a href="https://dagger.dev">Dagger</a>. For server code, common options include <a
 * href="https://github.com/google/guice/wiki/Motivation">Guice</a> and <a

   *
   * @since 19.0
   */
  public static HashFunction concatenating(
      HashFunction first, HashFunction second, HashFunction... rest) {
    // We can't use Lists.asList() here because there's no hash->collect dependency
    List<HashFunction> list = new ArrayList<>();
    list.add(first);
    list.add(second);
    Collections.addAll(list, rest);
    return new ConcatenatedHashFunction(list.toArray(new HashFunction[0]));
  }