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 when checking benchmarks and comparisons you should keep the following in mind.

## Benchmarks and speed

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]`** ✨.

                for label in pr.labels.nodes:
                    if label.name == "lang-all":
                        translation_reviewers[review.author.login] += 1
                        break
        for reviewer in pr_reviewers:
            reviewers[reviewer] += 1
        if pr.state == "MERGED" and pr.author:
            contributors[pr.author.login] += 1
    return ContributorsResults(

* 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:

## 0.106.0

### Breaking Changes

Using resources from dependencies with `yield` in background tasks is no longer supported.

This change is what supports the new features, read below. 🤓

### Dependencies with `yield`, `HTTPException` and Background Tasks

Dependencies with `yield` now can raise `HTTPException` and other exceptions after `yield`. 🎉

* The *path* can contain an <a href="https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#key-expression" class="external-link" target="_blank">OpenAPI 3 expression</a> (see more below) where it can use variables with parameters and parts of the original request sent to *your API*.

### The callback path expression

!!! note
    When defining a <a href="https://docs.pydantic.dev/latest/concepts/types/#unions" class="external-link" target="_blank">`Union`</a>, include the most specific type first, followed by the less specific type. In the example below, the more specific `PlaneItem` comes before `CarItem` in `Union[PlaneItem, CarItem]`.

=== "Python 3.10+"

    ```Python hl_lines="1  14-15  18-20  33"
    {!> ../../../docs_src/extra_models/tutorial003_py310.py!}