* この`dict`の値は`float`型です。（つまり、各項目の価格）

#### `Optional`

また、`Optional`を使用して、変数が`str`のような型を持つことを宣言することもできますが、それは「オプション」であり、`None`にすることもできます。

```Python hl_lines="1 4"
{!../../docs_src/python_types/tutorial009.py!}
```

ただの`str`の代わりに`Optional[str]`を使用することで、エディタは値が常に`str`であると仮定している場合に実際には`None`である可能性があるエラーを検出するのに役立ちます。

#### ジェネリック型

以下のように角括弧で型パラメータを取る型を:

* `List`
* `Tuple`
* `Set`
* `Dict`

	if d == nil || len(d.Cache) <= limit {
		return
	}
	top := hashPath(d.Info.Name).Key()
	topE := d.find(top)
	if topE == nil {
		scannerLogIf(GlobalContext, errors.New("forceCompact: root not found"))
		return
	}
	// If off by 2 orders of magnitude, compact self and log error.
	if len(topE.Children) > dataScannerForceCompactAtFolders {
		// If we still have too many children, compact self.

        sb.append("      id: ").append(getId()).append(LS);
        sb.append("      url: ").append(getUrl()).append(LS);
        sb.append("   layout: ").append(layout != null ? layout : "none");

        if (proxy != null) {
            sb.append(LS)
                    .append("    proxy: ")
                    .append(proxy.getHost())
                    .append(':')

    // --------------------------/
    // - Class/Member Variables -/
    // --------------------------/

    /**
     * The one-based line number. The value will be non-positive if
     * unknown.
     */
    private int lineNumber = -1;

    /**
     * The one-based column number. The value will be non-positive
     * if unknown.
     */
    private int columnNumber = -1;

    /**
     * Field source.

/// tip | Consejo

El segundo argumento de <a href="https://docs.python.org/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> es el valor por defecto a retornar.

Si no se proporciona, es `None` por defecto; aquí proporcionamos `"World"` como el valor por defecto para usar.

///

Luego podrías llamar a ese programa Python:

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

<div class="termy">

```console

 * Implementation detail for the internal structure of {@link Range} instances. Represents a unique
 * way of "cutting" a "number line" (actually of instances of type {@code C}, not necessarily
 * "numbers") into two sections; this can be done below a certain value, above a certain value,
 * below all values or above all values. With this object defined in this way, an interval can
 * always be represented by a pair of {@code Cut} instances.
 *

     *
     * - If the stack overflowed after set()/setValue() stored its result, then a call to
     * setException() will be a no-op because the Future is already done.
     *
     * Both scenarios are bad: The output Future might never complete, or, if it does complete, it
     * might not run some of its listeners. The likely result is that the app will hang. (And of

  }

  public void testToArray() {
    // need explicit type parameter to avoid javac warning!?
    List<Boolean> none = Arrays.<Boolean>asList();
    assertThat(Booleans.toArray(none)).isEqualTo(EMPTY);

    List<Boolean> one = Arrays.asList(false);
    assertThat(Booleans.toArray(one)).isEqualTo(ARRAY_FALSE);

    boolean[] array = {false, false, true};

    List<Boolean> three = Arrays.asList(false, false, true);

 * {@link UnsupportedOperationException} when they are not implemented. Most implementations should
 * support either all add operations or none of them, all removal operations or none of them, and if
 * and only if all of these are supported, the {@code setCount} methods as well.
 *
 * <p>A multiset uses {@link Object#equals} to determine whether two instances should be considered

	// test simultaneous reads
	n := 0
	done := make(chan bool)
	for i := 0; i < 5; i++ {
		for j, ft := range zt.File {
			go func(j int, ft ZipTestFile) {
				readTestFile(t, zt, ft, z.File[j], raw)
				done <- true
			}(j, ft)
			n++
		}
	}
	for ; n > 0; n-- {
		<-done
	}
}

func equalTimeAndZone(t1, t2 time.Time) bool {
	name1, offset1 := t1.Zone()
	name2, offset2 := t2.Zone()