}

  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() {

  }

  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() {

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: