* Data conversion
* Data validation
* Automatic documentation

## Special types and validation

Apart from normal singular types like `str`, `int`, `float`, etc. you can use more complex singular types that inherit from `str`.

### `Response`

The main `Response` class, all the other responses inherit from it.

You can return it directly.

It accepts the following parameters:

* `content` - A `str` or `bytes`.
* `status_code` - An `int` HTTP status code.
* `headers` - A `dict` of strings.

```Python hl_lines="11"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

### Create a `Base` class

Now we will use the function `declarative_base()` that returns a class.

Later we will inherit from this class to create each of the database models or classes (the ORM models):

```Python hl_lines="13"
{!../../../docs_src/sql_databases/sql_app/database.py!}
```

## Create the database models

    )
    assert response.status_code == 200


@needs_pydanticv1
def test_openapi_schema(client: TestClient):
    response = client.get("/openapi.json")
    assert response.status_code == 200, response.text
    # insert_assert(response.json())
    assert response.json() == {
        "openapi": "3.1.0",
        "info": {"title": "FastAPI", "version": "0.1.0"},
        "paths": {
            "/items/{item_id}": {
                "put": {

### Create an `Enum` class

Import `Enum` and create a sub-class that inherits from `str` and from `Enum`.

By inheriting from `str` the API docs will be able to know that the values must be of type `string` and will be able to render correctly.

```Python hl_lines="1"
{!../../../docs_src/first_steps/tutorial001.py!}
```

`FastAPI` is a Python class that provides all the functionality for your API.

!!! note "Technical Details"
    `FastAPI` is a class that inherits directly from `Starlette`.

    You can use all the <a href="https://www.starlette.io/" class="external-link" target="_blank">Starlette</a> functionality with `FastAPI` too.

### Step 2: create a `FastAPI` "instance"

        typer.echo(f"The language was already created: {lang}")
        raise typer.Abort()
    new_path.mkdir()
    new_config_path: Path = Path(new_path) / mkdocs_name
    new_config_path.write_text("INHERIT: ../en/mkdocs.yml\n", encoding="utf-8")
    new_config_docs_path: Path = new_path / "docs"
    new_config_docs_path.mkdir()
    en_index_path: Path = en_docs_path / "docs" / "index.md"

How does this work? Let's check that out. 🤓

### Type Annotations and Tooling

First let's see how editors, mypy and other tools would see this.

`BaseUser` has the base fields. Then `UserIn` inherits from `BaseUser` and adds the `password` field, so, it will include all the fields from both models.

We annotate the function return type as `BaseUser`, but we are actually returning a `UserIn` instance.

If you pass a custom list of `servers` and there's a `root_path` (because your API lives behind a proxy), **FastAPI** will insert a "server" with this `root_path` at the beginning of the list.

For example:

```Python hl_lines="4-7"
{!../../../docs_src/behind_a_proxy/tutorial003.py!}
```

Will generate an OpenAPI schema like:

!!! check "**FastAPI** uses it to"
    Handle all the core web parts. Adding features on top.

    The class `FastAPI` itself inherits directly from the class `Starlette`.

    So, anything that you can do with Starlette, you can do it directly with **FastAPI**, as it is basically Starlette on steroids.