**FastAPI** 會確保每個使用 `yield` 的相依，其「結束程式碼」會以正確的順序執行。

例如，`dependency_c` 可以相依於 `dependency_b`，而 `dependency_b` 相依於 `dependency_a`：

{* ../../docs_src/dependencies/tutorial008_an_py310.py hl[6,14,22] *}

而且它們都可以使用 `yield`。

在這個例子中，`dependency_c` 為了執行它的結束程式碼，需要來自 `dependency_b`（此處命名為 `dep_b`）的值仍然可用。

同理，`dependency_b` 為了執行它的結束程式碼，需要來自 `dependency_a`（此處命名為 `dep_a`）的值可用。

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a=Depends(dependency_a)):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b=Depends(dependency_b)):
    dep_c = generate_dep_c()
    try:
        yield dep_c

**FastAPI** se asegurará de que el "código de salida" en cada dependencia con `yield` se ejecute en el orden correcto.

Por ejemplo, `dependency_c` puede tener una dependencia de `dependency_b`, y `dependency_b` de `dependency_a`:

{* ../../docs_src/dependencies/tutorial008_an_py310.py hl[6,14,22] *}

Y todas ellas pueden usar `yield`.

Par exemple, `dependency_c` peut dépendre de `dependency_b`, et `dependency_b` de `dependency_a` :

{* ../../docs_src/dependencies/tutorial008_an_py310.py hl[6,14,22] *}

Et elles peuvent toutes utiliser `yield`.

Dans ce cas, `dependency_c`, pour exécuter son code de sortie, a besoin que la valeur de `dependency_b` (appelée ici `dep_b`) soit toujours disponible.

 *
 * @since 4.0.0
 */
@Experimental
@Immutable
public interface Dependency extends Artifact {
    /**
     * {@return the type of the dependency}.
     * A dependency can be a <abbr>JAR</abbr> file,
     * a modular-<abbr>JAR</abbr> if it is intended to be placed on the module path,
     * a <abbr>JAR</abbr> containing test classes, <i>etc.</i>

  <modelVersion>4.0.0</modelVersion>

  <groupId>ut.simple</groupId>
  <artifactId>dependency</artifactId>
  <version>1.0</version>

  <name>Simple Unit Test Dependency</name>

    private Project project;

    /**
     * <p>setUp.</p>
     */
    @BeforeEach
    void setUp() {
        Dependency dependency1 = new Dependency();
        dependency1.setArtifactId("dep1");
        Dependency dependency2 = new Dependency();
        dependency2.setArtifactId("dep2");

        project = new Project();
        project.setModelVersion("4.0.0");
        project.setGroupId("org.apache.maven");

        <version>1.0</version>
        <dependencies>
          <dependency>
            <groupId>test</groupId>
            <artifactId>a</artifactId>
            <version>0.1</version>
          </dependency>
          <dependency>
            <groupId>test</groupId>
            <artifactId>b</artifactId>
            <version>${missing.property}</version>
          </dependency>
        </dependencies>
      </plugin>
    </plugins>

                + " -Dversion=aVersion -Dclassifier=aClassifier -Dpackaging=jar -Dfile=/path/to/file"
                + " -Durl=[url] -DrepositoryId=[id]" + LS + LS + "  Path to dependency: " + LS + "  \t1) dependency1"
                + LS + "  \t2) dependency2" + LS + LS;
        String actual = AbstractArtifactResolutionException.constructMissingArtifactMessage(

  <artifactId>foo</artifactId>
  <groupId>foo</groupId>
  <version>99.44</version>
  <packaging>bleh</packaging>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <artifactId>artifactId</artifactId>
        <groupId>groupId</groupId>
      </dependency>
    </dependencies>
  </dependencyManagement>