An example could be that you have an external authentication provider that you need to call.

You send it a token and it returns an authenticated user.

This provider might be charging you per request, and calling it might take some extra time than if you had a fixed mock user for tests.

You probably want to test the external provider once, but not necessarily call it for every test that runs.

`HTTPException` is a normal Python exception with additional data relevant for APIs.

Because it's a Python exception, you don't `return` it, you `raise` it.

```Python
class Cat:
    def __init__(self, name: str):
        self.name = name


fluffy = Cat(name="Mr Fluffy")
```

In this case, `fluffy` is an instance of the class `Cat`.

And to create `fluffy`, you are "calling" `Cat`.

So, a Python class is also a **callable**.

Then, in **FastAPI**, you could use a Python class as a dependency.

Sonrasında ise projenin odağı değişti.

Geliştiricinin Starlette'e odaklanması gerekince proje de artık bir API web framework'ü olmayı bıraktı.

Artık APIStar, OpenAPI özelliklerini doğrulamak için bir dizi araç sunan bir proje haline geldi.

!!! info "Bilgi"
    APIStar, aşağıdaki projeleri de üreten Tom Christie tarafından geliştirildi:

    * Django REST Framework
    * **FastAPI**'ın da dayandığı Starlette

{!../../../docs_src/custom_response/tutorial004.py!}
```

In this example, the function `generate_html_response()` already generates and returns a `Response` instead of returning the HTML in a `str`.

By returning the result of calling `generate_html_response()`, you are already returning a `Response` that will override the default **FastAPI** behavior.

"_Python ile kullanıma hazır bir API oluşturmak isteyen herhangi biri için, **FastAPI**'ı şiddetle tavsiye ederim. **Harika tasarlanmış**, **kullanımı kolay** ve **yüksek ölçeklenebilir**, API odaklı geliştirme stratejimizin **ana bileşeni** haline geldi ve Virtual TAC Engineer gibi birçok otomasyon ve servisi yönetiyor._"

Entonces, relacionado con la paradoja del huevo y la gallina, ¿cómo se llama a la primera función `async`?

Si estás trabajando con **FastAPI** no tienes que preocuparte por eso, porque esa "primera" función será tu *path operation function*, y FastAPI sabrá cómo hacer lo pertinente.

    But **FastAPI** will know that it has to solve `query_extractor` first, to pass the results of that to `query_or_cookie_extractor` while calling it.

```mermaid
graph TB

query_extractor(["query_extractor"])
query_or_cookie_extractor(["query_or_cookie_extractor"])

read_query["/items/"]

    ```Python hl_lines="5  22"
    {!> ../../../docs_src/encoder/tutorial001.py!}
    ```

In this example, it would convert the Pydantic model to a `dict`, and the `datetime` to a `str`.

The result of calling it is something that can be encoded with the Python standard <a href="https://docs.python.org/3/library/json.html#json.dumps" class="external-link" target="_blank">`json.dumps()`</a>.

!!! tip
    You'll see what other "things", apart from functions, can be used as dependencies in the next chapter.

Whenever a new request arrives, **FastAPI** will take care of:

* Calling your dependency ("dependable") function with the correct parameters.
* Get the result from your function.
* Assign that result to the parameter in your *path operation function*.

```mermaid
graph TB