## Dependencies

We see that we are going to need some dependencies used in several places of the application.

So we put them in their own `dependencies` module (`app/dependencies.py`).

We will now use a simple dependency to read a custom `X-Token` header:

=== "Python 3.9+"

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

from ...utils import needs_py310


@needs_py310
def test_override_in_items_run():
    from docs_src.dependency_testing.tutorial001_an_py310 import test_override_in_items

    test_override_in_items()


@needs_py310
def test_override_in_items_with_q_run():
    from docs_src.dependency_testing.tutorial001_an_py310 import (
        test_override_in_items_with_q,
    )

    test_override_in_items_with_q()


@needs_py310

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: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):
    dep_c = generate_dep_c()

    ```

And that way we are able to "parameterize" our dependency, that now has `"bar"` inside of it, as the attribute `checker.fixed_content`.

## Use the instance as a dependency

Then, we could use this `checker` in a `Depends(checker)`, instead of `Depends(FixedContentQueryChecker)`, because the dependency is the instance, `checker`, not the class itself.

And when solving the dependency, **FastAPI** will call this `checker` like:

```Python

**FastAPI** has a very powerful but intuitive **<abbr title="also known as components, resources, providers, services, injectables">Dependency Injection</abbr>** system.

It is designed to be very simple to use, and to make it very easy for any developer to integrate other components with **FastAPI**.

## What is "Dependency Injection"

# Dependencies in path operation decorators

In some cases you don't really need the return value of a dependency inside your *path operation function*.

Or the dependency doesn't return a value.

But you still need it to be executed/solved.

For those cases, instead of declaring a *path operation function* parameter with `Depends`, you can add a `list` of `dependencies` to the *path operation decorator*.

    return {"message": "Hello Users!", "params": commons}


client = TestClient(app)


async def override_dependency(q: Union[str, None] = None):
    return {"q": q, "skip": 5, "limit": 10}


app.dependency_overrides[common_parameters] = override_dependency


def test_override_in_items():
    response = client.get("/items/")
    assert response.status_code == 200
    assert response.json() == {

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

In this case, you can override the dependency that calls that provider, and use a custom dependency that returns a mock user, only for your tests.

### Use the `app.dependency_overrides` attribute

For these cases, your **FastAPI** application has an attribute `app.dependency_overrides`, it is a simple `dict`.

    * Required scopes `["items"]` with the dependency:
    * `get_current_active_user`:
        *  The dependency function `get_current_active_user` has:
            * Required scopes `["me"]` with the dependency:
            * `get_current_user`:
                * The dependency function `get_current_user` has:
                    * No scopes required by itself.
                    * A dependency using `oauth2_scheme`.