* its runtime check.
   *
   * <p>An example use case for this method is in implementing an {@code Iterator<T>} whose {@code
   * next} field is lazily initialized. The type of that field would be {@code @Nullable T}, and the
   * code would be responsible for populating a "real" {@code T} (which might still be the value
   * {@code null}!) before returning it to callers. Depending on how the code is structured, a

* **`200 - 299`** are for "Successful" responses. These are the ones you would use the most.
    * `200` is the default status code, which means everything was "OK".
    * Another example would be `201`, "Created". It is commonly used after creating a new record in the database.

You import it and create an "instance" the same way you would with the class `FastAPI`:

{* ../../docs_src/bigger_applications/app_an_py39/routers/users.py hl[1,3] title["app/routers/users.py"] *}

### *Path operations* with `APIRouter` { #path-operations-with-apirouter }

And then you use it to declare your *path operations*.

Use it the same way you would use the `FastAPI` class:

     * to "pull in" transitive dependencies of eliminated projects, as for case above, the properly filtered list would
     * be {@code a -> c}.
     * <p>
     * Original code would falsely report {@code a} project as "without dependencies", basically would lose link due
     * filtering. This causes build ordering issues in concurrent builders.
     */
    private List<MavenProject> applyFilter(

   * time, then a request arrived and was immediately granted? This RateLimiter would immediately
   * forget about that past underutilization. This may result in either underutilization or
   * overflow, depending on the real world consequences of not using the expected rate.
   *
   * Past underutilization could mean that excess resources are available. Then, the RateLimiter

        "needy"
      ],
      "msg": "Field required",
      "input": null
    }
  ]
}
```

As `needy` is a required parameter, you would need to set it in the URL:

```
http://127.0.0.1:8000/items/foo-item?needy=sooooneedy
```

...this would work:

```JSON
{
    "item_id": "foo-item",
    "needy": "sooooneedy"
}
```

/// note

A path parameter is always required as it has to be part of the path. Even if you declared it with `None` or set a default value, it would not affect anything, it would still be always required.

///

## Order the parameters as you need { #order-the-parameters-as-you-need }

/// tip

This is probably not as important or necessary if you use `Annotated`.

///

```Python hl_lines="1  4"
{!> ../../docs_src/python_types/tutorial008b_py39.py!}
```

////

In both cases this means that `item` could be an `int` or a `str`.

#### Possibly `None` { #possibly-none }

You can declare that a value could have a type, like `str`, but that it could also be `None`.

In Python 3.6 and above (including Python 3.10) you can declare it by importing and using `Optional` from the `typing` module.

 * }
 * }
 *
 * @author Ben Yu
 * @since 12.0
 */
/*
 * A nullable bound would let users create a TypeParameter instance for a parameter with a nullable
 * bound. However, it would also let them create `new TypeParameter<@Nullable T>() {}`, which
 * wouldn't behave as users might expect. Additionally, it's not clear how the TypeToken API could
 * support even a "normal" `TypeParameter<T>` when `<T>` has a nullable bound. (See the discussion

      // the operation would be a no-op, so exceptions are allowed but not required
    }
    expectUnchanged();
  }

  @MapFeature.Require(absent = SUPPORTS_PUT)
  public void testReplaceEntry_unsupportedAbsentKey() {
    try {
      getMap().replace(k3(), v3(), v4());
    } catch (UnsupportedOperationException tolerated) {
      // the operation would be a no-op, so exceptions are allowed but not required