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] *}

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)

{* ../../docs_src/settings/app02_an_py39/test_main.py hl[9:10,13,21] *}

In the dependency override we set a new value for the `admin_email` when creating the new `Settings` object, and then we return that new object.

Then we can test that it is used.

## Reading a `.env` file { #reading-a-env-file }

@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.

// of the payload and verify that it matches the expected signature value.
//
// The last chunk is *always* 0-sized. So, we must only return io.EOF if we have encountered
// a chunk with a chunk size = 0. However, this chunk still has a signature and we must
// verify it.
const maxChunkSize = 16 << 20 // 16 MiB

// Read - implements `io.Reader`, which transparently decodes

		}
		f.Write([]byte(text))
		_, err := f.Finish()
		// We should get a no exit code error
		if err == nil || !wantErr.MatchString(err.Error()) {
			t.Errorf("want error matching %s, got %s", wantErr, err)
		}
		// And it should flush all output (even if it looks
		// like we may be getting an exit code)
		if got := out.String(); text != got {

		// https://en.wikipedia.org/wiki/Copy_protection so the incoming stream
		// is always going to be in-memory as we cannot re-read from what we
		// wrote to disk - since that amounts to "copying" from a "copy"
		// instead of "copying" from source, we need the stream to be seekable
		// to ensure that we can make fan-out calls concurrently.
		buf := bytebufferpool.Get()
		defer func() {
			buf.Reset()