func (s *Singleton[T]) IsSet() bool {
	select {
	case <-s.set:
		return true
	default:
		return false
	}
}

// Set the value and unblock all Get requests.
// This may only be called once, a second call will panic.
func (s *Singleton[T]) Set(v *T) {
	s.v = v
	close(s.set)

    // JVM shutdown will not be prevented from exiting after this service has stopped or failed.
    // Technically this listener is added after start() was called so it is a little gross, but it
    // is called within doStart() so we know that the service cannot terminate or fail concurrently

# Python Types Intro { #python-types-intro }

Python has support for optional "type hints" (also called "type annotations").

These **"type hints"** or annotations are a special syntax that allow declaring the <abbr title="for example: str, int, float, bool">type</abbr> of a variable.

By declaring types for your variables, editors and tools can give you better support.

 *
 * <p>The service is called during model building for all model objects, allowing
 * implementations to decide which objects to process and how to optimize them.</p>
 *
 * @since 4.0.0
 */
public interface ModelObjectProcessor {

    /**
     * Process a model object, potentially returning a pooled or optimized version.
     *
     * <p>This method is called during model building for various model objects.

## Call `uvicorn` { #call-uvicorn }

In your FastAPI application, import and run `uvicorn` directly:

{* ../../docs_src/debugging/tutorial001_py39.py hl[1,15] *}

### About `__name__ == "__main__"` { #about-name-main }

The main purpose of the `__name__ == "__main__"` is to have some code that is executed when your file is called with:

<div class="termy">

```console

      /*
       * requireNonNull is safe because we don't null out `sequencer` except:
       *
       * - after the requireNonNull call below. (And this object has its Runnable.run override
       *   called only once, just as it has its Executor.execute override called only once.)
       *
       * - if we return immediately from `execute` (in which case we never get here)
       *

For example, you could create a project called `philosophers-stone`, this program depends on another package called **`harry`, using the version `1`**. So, you need to install `harry`.

```mermaid
flowchart LR
    stone(philosophers-stone) -->|requires| harry-1[harry v1]
```

  }

  public void testFutureGetThrowsWrappedError() throws Exception {
    Error error = new Error();
    inputFuture.setException(error);
    // Verify that get throws an ExecutionException, caused by an Error, when
    // the callback is called.
    listener.assertException(error);
  }

  public void testAddListenerAfterCallback() throws Throwable {
    inputFuture.set(VALID_INPUT_DATA);

   * network include {@code node}.
   *
   * <p>This is equal to the union of {@link #inEdges(Object)} and {@link #outEdges(Object)}.
   *
   * <p>If {@code node} is removed from the network after this method is called, the {@code Set}
   * {@code view} returned by this method will be invalidated, and will throw {@code
   * IllegalStateException} if it is accessed in any way, with the following exceptions:
   *
   * <ul>

 *
 * <p>{@code CharSource} provides two kinds of methods:
 *
 * <ul>
 *   <li><b>Methods that return a reader:</b> These methods should return a <i>new</i>, independent
 *       instance each time they are called. The caller is responsible for ensuring that the
 *       returned reader is closed.
 *   <li><b>Convenience methods:</b> These are implementations of common operations that are