You can define dependencies that in turn can define dependencies themselves.

In the end, a hierarchical tree of dependencies is built, and the **Dependency Injection** system takes care of solving all these dependencies for you (and their sub-dependencies) and providing (injecting) the results at each step.

                if dependencies:
                    current_dependencies.extend(dependencies)
                if route.dependencies:
                    current_dependencies.extend(route.dependencies)
                self.add_api_websocket_route(
                    prefix + route.path,
                    route.endpoint,
                    dependencies=current_dependencies,

  # Find all one-step dependencies of those tests which are from //tensorflow
  # (since external deps will come from Python-level pip dependencies),
  # excluding dependencies and files that are known to be unneccessary.
  # This creates a list of targets under //tensorflow that are required for
  # TensorFlow python tests.

from fastapi import Depends, FastAPI

from .dependencies import get_query_token, get_token_header
from .internal import admin
from .routers import items, users

app = FastAPI(dependencies=[Depends(get_query_token)])


app.include_router(users.router)
app.include_router(items.router)
app.include_router(
    admin.router,
    prefix="/admin",
    tags=["admin"],
    dependencies=[Depends(get_token_header)],

Also fügen wir sie in ihr eigenes `dependencies`-Modul (`app/dependencies.py`) ein.

Wir werden nun eine einfache Abhängigkeit verwenden, um einen benutzerdefinierten `X-Token`-Header zu lesen:

//// tab | Python 3.9+

```Python hl_lines="3  6-8" title="app/dependencies.py"
{!> ../../docs_src/bigger_applications/app_an_py39/dependencies.py!}
```

////

//// tab | Python 3.8+

    - tutorial/body-updates.md
    - Dependencies:
      - tutorial/dependencies/index.md
      - tutorial/dependencies/classes-as-dependencies.md
      - tutorial/dependencies/sub-dependencies.md
      - tutorial/dependencies/dependencies-in-path-operation-decorators.md
      - tutorial/dependencies/global-dependencies.md
      - tutorial/dependencies/dependencies-with-yield.md
    - Security:

def test_internal_error(client: TestClient):
    from docs_src.dependencies.tutorial008d_an import InternalError

    with pytest.raises(InternalError) as exc_info:
        client.get("/items/portal-gun")
    assert (
        exc_info.value.args[0] == "The portal gun is too dangerous to be owned by Rick"
    )


def test_internal_server_error():
    from docs_src.dependencies.tutorial008d_an import app

//// tab | Python 3.10+

```Python hl_lines="8-9"
{!> ../../docs_src/dependencies/tutorial001_an_py310.py!}
```

////

//// tab | Python 3.9+

```Python hl_lines="8-11"
{!> ../../docs_src/dependencies/tutorial001_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="9-12"
{!> ../../docs_src/dependencies/tutorial001_an.py!}
```

////

//// tab | Python 3.10+ non-Annotated

Então, as colocamos em seu próprio módulo de `dependencies` (`app/dependencies.py`).

Agora usaremos uma dependência simples para ler um cabeçalho `X-Token` personalizado:

//// tab | Python 3.9+

```Python hl_lines="3  6-8" title="app/dependencies.py"
{!> ../../docs_src/bigger_applications/app_an_py39/dependencies.py!}
```

////

//// tab | Python 3.8+

### Injection de dépendances

FastAPI contient un système simple mais extrêmement puissant d'<abbr title='aussi connus sous le nom de "composants", "ressources", "services", "providers"'><strong>Injection de Dépendances</strong></abbr>.

* Même les dépendances peuvent avoir des dépendances, créant une hiérarchie ou un **"graph" de dépendances**
* Tout est **automatiquement géré** par le framework