# CORS (Cross-Origin Resource Sharing)

<a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS" class="external-link" target="_blank">CORS ou "Cross-Origin Resource Sharing"</a> refere-se às situações em que um frontend rodando em um navegador possui um código JavaScript que se comunica com um backend, e o backend está em uma "origem" diferente do frontend.

## Origem

Pydantic, which is what powers all the data validation and serialization in FastAPI, has a special behavior when you use `Optional` or `Union[Something, None]` without a default value, you can read more about it in the Pydantic docs about <a href="https://docs.pydantic.dev/2.3/usage/models/#required-optional-fields" class="external-link" target="_blank">Required fields</a>.

///

/// tip

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package gradlebuild.docs.dsl.docbook;

import gradlebuild.docs.dsl.docbook.model.ClassExtensionDoc;
import org.w3c.dom.Document;
import org.w3c.dom.Element;

public class ExtensionMethodsSummaryRenderer {
    private final MethodTableRenderer methodTableRenderer;

	// https://istio.io/docs/setup/platform-setup/
	MinK8SVersion               = 28
	UnSupportedK8SVersionLogMsg = "\nThe Kubernetes version %s is not supported by Istio %s. The minimum supported Kubernetes version is 1.%d.\n" +
		"Proceeding with the installation, but you might experience problems. " +
		"See https://istio.io/latest/docs/releases/supported-releases/ for a list of supported versions.\n"
)

///

## Check the automatic API docs

Now, run your app:

<div class="termy">

```console
$ fastapi dev app/main.py

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
```

</div>

And open the docs at <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.

    * В запросах и ответах будет представлен в виде общего количества секунд типа `float`.
    * Pydantic также позволяет представить его как "Кодировку разницы во времени ISO 8601", <a href="https://docs.pydantic.dev/latest/concepts/serialization/#json_encoders" class="external-link" target="_blank">см. документацию для получения дополнительной информации</a>.
* `frozenset`:
    * В запросах и ответах обрабатывается так же, как и `set`:

	expectPodAddedToIPSet(fakeIPSetDeps, podIP, pod.ObjectMeta)

	err := m.AddPodToMesh(fakeCtx, pod, podIPs, "")
	assert.NoError(t, err)

	fakeIPSetDeps.AssertExpectations(t)

	pod, err = fakeClientSet.CoreV1().Pods("test").Get(fakeCtx, "test", metav1.GetOptions{})
	assert.NoError(t, err)
	assert.Equal(t, len(pod.Annotations), 1)
	assert.Equal(t, pod.Annotations[annotation.AmbientRedirection.Name], constants.AmbientRedirectionEnabled)
}

So, the line:

```Python
    uvicorn.run(app, host="0.0.0.0", port=8000)
```

will not be executed.

/// info

For more information, check <a href="https://docs.python.org/3/library/__main__.html" class="external-link" target="_blank">the official Python docs</a>.

///

## Run your code with your debugger

```

////

Notice that the *path operations* define the models they use for request payload and response payload, using the models `Item` and `ResponseMessage`.

### API Docs

If you go to the API docs, you will see that it has the **schemas** for the data to be sent in requests and received in responses:

<img src="/img/tutorial/generate-clients/image01.png">

`@lru_cache` is part of `functools` which is part of Python's standard library, you can read more about it in the <a href="https://docs.python.org/3/library/functools.html#functools.lru_cache" class="external-link" target="_blank">Python docs for `@lru_cache`</a>.

## Recap

You can use Pydantic Settings to handle the settings or configurations for your application, with all the power of Pydantic models.