def test_query_list_empty():
    response = client.get("/query/list/")
    assert response.status_code == 422


def test_query_list_default():
    response = client.get("/query/list-default/?device_ids=1&device_ids=2")
    assert response.status_code == 200
    assert response.json() == [1, 2]


def test_query_list_default_empty():
    response = client.get("/query/list-default/")

    ```

* `description: Union[str, None] = None` (or `str | None = None` in Python 3.10) has a default of `None`.
* `tax: float = 10.5` has a default of `10.5`.
* `tags: List[str] = []` has a default of an empty list: `[]`.

but you might want to omit them from the result if they were not actually stored.

    For example, in `app/main.py` you could have a line like:

    ```
    from app.routers import items
    ```

* The `app` directory contains everything. And it has an empty file `app/__init__.py`, so it is a "Python package" (a collection of "Python modules"): `app`.

def get_typed_return_annotation(call: Callable[..., Any]) -> Any:
    signature = inspect.signature(call)
    annotation = signature.return_annotation

    if annotation is inspect.Signature.empty:
        return None

    globalns = getattr(call, "__globals__", {})
    return get_typed_annotation(annotation, globalns)


def get_dependant(
    *,
    path: str,
    call: Callable[..., Any],

```Python hl_lines="23-29"
{!../../../docs_src/sql_databases_peewee/sql_app/main.py!}
```

Here we have an empty `yield` because we are actually not using the database object directly.

```
.
└── sql_app
    ├── __init__.py
    ├── crud.py
    ├── database.py
    ├── main.py
    ├── models.py
    └── schemas.py
```

The file `__init__.py` is just an empty file, but it tells Python that `sql_app` with all its modules (Python files) is a package.

Now let's see what each file/module does.

## Install `SQLAlchemy`

First you need to install `SQLAlchemy`:

```

</div>

!!! info
    There are other formats and tools to define and install package dependencies.

### Create the **FastAPI** Code

* Create an `app` directory and enter it.
* Create an empty file `__init__.py`.
* Create a `main.py` file with:

```Python
from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():