You can create dependencies that have **sub-dependencies**.

They can be as **deep** as you need them to be.

**FastAPI** will take care of solving them.

## First dependency "dependable" { #first-dependency-dependable }

You could create a first dependency ("dependable") like:

{* ../../docs_src/dependencies/tutorial005_an_py310.py hl[8:9] *}

It declares an optional query parameter `q` as a `str`, and then it just returns it.

Let's see a very simple example. It will be so simple that it is not very useful, for now.

But this way we can focus on how the **Dependency Injection** system works.

### Create a dependency, or "dependable" { #create-a-dependency-or-dependable }

Let's first focus on the dependency.

It is just a function that can take all the same parameters that a *path operation function* can take:

## A `dict` from the previous example { #a-dict-from-the-previous-example }

In the previous example, we were returning a `dict` from our dependency ("dependable"):

{* ../../docs_src/dependencies/tutorial001_an_py310.py hl[9] *}

But then we get a `dict` in the parameter `commons` of the *path operation function*.

Using `BackgroundTasks` also works with the dependency injection system, you can declare a parameter of type `BackgroundTasks` at multiple levels: in a *path operation function*, in a dependency (dependable), in a sub-dependency, etc.

**FastAPI** knows what to do in each case and how to reuse the same object, so that all the background tasks are merged together and are run in the background afterwards:

* ⬆ Bump dawidd6/action-download-artifact from 2.9.0 to 2.21.1. PR [#5130](https://github.com/tiangolo/fastapi/pull/5130) by [@dependabot[bot]](https://github.com/apps/dependabot).
* ⬆ Bump actions/setup-python from 2 to 4. PR [#5129](https://github.com/tiangolo/fastapi/pull/5129) by [@dependabot[bot]](https://github.com/apps/dependabot).

Here **FastAPI** won't get confused because you are using `Depends`.

///

/// check

The way this dependency system is designed allows us to have different dependencies (different "dependables") that all return a `User` model.

We are not restricted to having only one dependency that can return that type of data.

///

## Other models { #other-models }

  count: 776
  avatarUrl: https://avatars.githubusercontent.com/u/1326112?u=cb5d06e73a9e1998141b1641aa88e443c6717651&v=4
  url: https://github.com/tiangolo
dependabot:
  login: dependabot
  count: 113
  avatarUrl: https://avatars.githubusercontent.com/in/29110?v=4
  url: https://github.com/apps/dependabot
alejsdev:
  login: alejsdev
  count: 48
  avatarUrl: https://avatars.githubusercontent.com/u/90076947?u=638c65283ac9e9e2c3a0f9d1e3370db4b8a2c58d&v=4

* L'utilisation de **contraintes de validation** comme `maximum_length` ou `regex`.
* Un **<abbr title="aussi connu sous le nom de composants, ressources, fournisseurs, services, injectables">système d'injection de dépendance </abbr>** très puissant et facile à utiliser .

  }

  public <T extends Readable & Appendable> void testMultiBound() {
    assertAssignable(new TypeToken<List<T>>() {}, new TypeToken<List<? extends Readable>>() {});
    assertAssignable(new TypeToken<List<T>>() {}, new TypeToken<List<? extends Appendable>>() {});
  }

  public void testToGenericType() {

### Dépendances

La même chose s'applique aux dépendances. Si une dépendance est définie avec `def` plutôt que `async def`, elle est exécutée dans la threadpool externe.

### Sous-dépendances