# Dependencies - `Depends()` and `Security()`

## `Depends()`

Dependencies are handled mainly with the special function `Depends()` that takes a callable.

Here is the reference for it and its parameters.

You can import it directly from `fastapi`:

```python
from fastapi import Depends
```

::: fastapi.Depends

## `Security()`

Nils Lindemann <******@****.***> 1711822605 +0100

Sebastián Ramírez <******@****.***> 1679142599 +0100

Sebastián Ramírez <******@****.***> 1652485102 -0500

Sebastián Ramírez <******@****.***> 1679142599 +0100

    {!> ../../../docs_src/dependencies/tutorial012_an.py!}
    ```

=== "Python 3.8 nicht annotiert"

    !!! tip "Tipp"
        Bevorzugen Sie die `Annotated`-Version, falls möglich.

    ```Python hl_lines="15"
    {!> ../../../docs_src/dependencies/tutorial012.py!}
    ```

    ```

=== "Python 3.8 Annotated가 없는 경우"

    !!! tip "팁"
        가능하다면 `Annotated`가 달린 버전을 권장합니다.

    ```Python hl_lines="15"
    {!> ../../../docs_src/dependencies/tutorial012.py!}
    ```

그리고 [*경로 작동 데코레이터*에 `dependencies` 추가하기](dependencies-in-path-operation-decorators.md){.internal-link target=_blank}에 대한 아이디어는 여전히 적용되지만 여기에서는 앱에 있는 모든 *경로 작동*에 적용됩니다.

## *경로 작동* 모음에 대한 의존성

### 第一层依赖项

下列代码创建了第一层依赖项：

```Python hl_lines="8-9"
{!../../../docs_src/dependencies/tutorial005.py!}
```

这段代码声明了类型为 `str` 的可选查询参数 `q`，然后返回这个查询参数。

这个函数很简单（不过也没什么用），但却有助于让我们专注于了解子依赖项的工作方式。

### 第二层依赖项

接下来，创建另一个依赖项函数，并同时用该依赖项自身再声明一个依赖项（所以这也是一个「依赖项」）：

```Python hl_lines="13"
{!../../../docs_src/dependencies/tutorial005.py!}
```

这里重点说明一下声明的参数：

* 尽管该函数自身是依赖项，但还声明了另一个依赖项（它「依赖」于其他对象）

# Global Dependencies

For some types of applications you might want to add dependencies to the whole application.

Similar to the way you can [add `dependencies` to the *path operation decorators*](dependencies-in-path-operation-decorators.md){.internal-link target=_blank}, you can add them to the `FastAPI` application.

In that case, they will be applied to all the *path operations* in the application:

=== "Python 3.9+"

    ```Python hl_lines="16"

# Testing Dependencies with Overrides

## Overriding dependencies during testing

There are some scenarios where you might want to override a dependency during testing.

You don't want the original dependency to run (nor any of the sub-dependencies it might have).