If not provided, it's `None` by default, here we provide `"World"` as the default value to use.

///

Then you could call that Python program:

//// tab | Linux, macOS, Windows Bash

<div class="termy">

```console
// Here we don't set the env var yet
$ python main.py

// As we didn't set the env var, we get the default value

Hello World from Python

// But if we create an environment variable first

In our code example above, we don't use it directly, but we pass it to FastAPI for it to use it.

The `lifespan` parameter of the `FastAPI` app takes an **async context manager**, so we can pass our new `lifespan` async context manager to it.

{* ../../docs_src/events/tutorial003_py39.py hl[22] *}

    <mockito.version>4.11.0</mockito.version>
    <plexus-io.version>3.5.1</plexus-io.version> <!-- We need at least 3.5.1 to avoid https://github.com/codehaus-plexus/plexus-io/issues/109. Once we upgrade maven-jar-plugin itself to a version new enough to depend on 3.5.1 or higher, we can remove this override. -->
    <toolchains-maven-plugin.version>4.5.0</toolchains-maven-plugin.version>
    <truth.version>1.4.5</truth.version>

	go func() {
		updateTicker := time.NewTicker(30 * time.Second)
		defer updateTicker.Stop()
		var lastUpdate time.Time

		// We need to merge since we will get the same buckets from each pool.
		// Therefore to get the exact bucket sizes we must merge before we can convert.
		var allMerged dataUsageCache

		update := func() {
			mu.Lock()
			defer mu.Unlock()

	}

	cache := store.lock()
	defer store.unlock()

	// We should only update the in-memory cache if there were no changes
	// to the in-memory cache since the disk loading began. If there
	// were changes to the in-memory cache we should wait for the next
	// cycle until we can safely update the in-memory cache.
	//
	// An in-memory cache must be replaced only if we know for sure that the

Let's look at how we can make that work.

## pytest.mark.anyio { #pytest-mark-anyio }

     * StackOverflowError: We can't reliably call setException(error).
     *
     * - Any kind of Error from a listener. Even if we could distinguish that case (by exposing some
     * extra state from AbstractFuture), our options are limited: A call to setException() would be
     * a no-op. We could log, but if that's what we really want, we should modify

    }

    @Override
    @ParametricNullness
    // We set the field only once (during construction or deserialization).
    @SuppressWarnings("SynchronizeOnNonFinalField")
    public T get() {
      // Another variant of Double Checked Locking.
      //
      // We use two volatile reads. We could reduce this to one by
      // putting our fields into a holder class, but (at least on x86)

@SuppressWarnings("serial") // we're overriding default serialization
// TODO(kevinb): I think we should push everything down to "BaseImmutableCollection" or something,
// just to do everything we can to emphasize the "practically an interface" nature of this class.
public abstract class ImmutableCollection<E> extends AbstractCollection<E> implements Serializable {
  /*
   * We expect SIZED (and SUBSIZED, if applicable) to be added by the spliterator factory methods.

         * ours. Therefore, we have to be careful about whether we create our own TypeVariable:
         *
         * 2a. If the resolved types are identical to the original types, then we can return the
         * original, identical JDK TypeVariable. By doing so, we sidestep the problem entirely.
         *