You put it on top of a function. Like a pretty decorative hat (I guess that's where the term came from).

    A "decorator" takes the function below and does something with it.

    In our case, this decorator tells **FastAPI** that the function below corresponds to the **path** `/` with an **operation** `get`.

    It is the "**path operation decorator**".

You can also use the other operations:

* `@app.post()`

But most importantly:

* It will **limit and filter** the output data to what is defined in the return type.
    * This is particularly important for **security**, we'll see more of that below.

## `response_model` Parameter

There are some cases where you need or want to return some data that is not exactly what the type declares.

=== "Python 3.8+ alternative"

    ```Python hl_lines="1  4"
    {!> ../../../docs_src/python_types/tutorial009b.py!}
    ```

#### Using `Union` or `Optional`

If you are using a Python version below 3.10, here's a tip from my very **subjective** point of view:

* 🚨 Avoid using `Optional[SomeType]`
* Instead ✨ **use `Union[SomeType, None]`** ✨.

    * They **expire**.
    * And then they need to be **renewed**, **acquired again** from the third party.
* The encryption of the connection happens at the **TCP level**.
    * That's one layer **below HTTP**.
    * So, the **certificate and encryption** handling is done **before HTTP**.
* **TCP doesn't know about "domains"**. Only about IP addresses.

* Kubernetes with an Ingress Controller like Nginx
    * With an external component like cert-manager for certificate renewals
* Handled internally by a cloud provider as part of their services (read below 👇)

{!../../../docs_src/bigger_applications/app/routers/items.py!}
```

#### How relative imports work

!!! tip
    If you know perfectly how imports work, continue to the next section below.

A single dot `.`, like in:

```Python
from .dependencies import get_token_header
```

would mean:

## Environment Variables

!!! tip
    If you already know what "environment variables" are and how to use them, feel free to skip to the next section below.

* `Annotated` from `typing` (or from `typing_extensions` in Python below 3.9)

=== "Python 3.10+"

    In Python 3.9 or above, `Annotated` is part of the standard library, so you can import it from `typing`.

    ```Python hl_lines="1  3"
    {!> ../../../docs_src/query_params_str_validations/tutorial002_an_py310.py!}
    ```

=== "Python 3.8+"

    In versions of Python below Python 3.9 you import `Annotated` from `typing_extensions`.

    else_: Optional["SchemaOrBool"] = Field(default=None, alias="else")
    dependentSchemas: Optional[Dict[str, "SchemaOrBool"]] = None
    prefixItems: Optional[List["SchemaOrBool"]] = None
    # TODO: uncomment and remove below when deprecating Pydantic v1
    # It generales a list of schemas for tuples, before prefixItems was available
    # items: Optional["SchemaOrBool"] = None
    items: Optional[Union["SchemaOrBool", List["SchemaOrBool"]]] = None

When creating a **FastAPI** class instance or an `APIRouter` you can specify which response class to use by default.

The parameter that defines this is `default_response_class`.

In the example below, **FastAPI** will use `ORJSONResponse` by default, in all *path operations*, instead of `JSONResponse`.

```Python hl_lines="2  4"
{!../../../docs_src/custom_response/tutorial010.py!}
```

!!! tip