...

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

* Send the invoice to some customer of the external developer.
* Collect the money.
* Send a notification back to the API user (the external developer).

* **`300`** e acima são para "Redirecionamento". As respostas com esses códigos de status podem ou não ter um corpo, exceto `304`, "Não modificado", que não deve ter um.
* **`400`** e acima são para respostas de "Erro do cliente". Este é o segundo tipo que você provavelmente mais usaria.
    * Um exemplo é `404`, para uma resposta "Não encontrado".
    * Para erros genéricos do cliente, você pode usar apenas `400`.

    username: str
    monthly_fee: float
    start_date: datetime


@app.webhooks.post("new-subscription")
def new_subscription(body: Subscription):
    """
    When a new user subscribes to your service we'll send you a POST request with this
    data to the URL that you register for the event `new-subscription` in the dashboard.
    """


@app.get("/users/")
def read_users():

@app.get("/query")
def read_query(q: Union[str, None]):
    return q


@app.get("/explicit-query")
def read_explicit_query(q: Union[str, None] = Query()):
    return q


@app.post("/body-embed")
def send_body_embed(b: Union[str, None] = Body(embed=True)):
    return b


client = TestClient(app)


def test_required_nonable_query_invalid():
    response = client.get("/query")

    ```

It declares an optional query parameter `q` as a `str`, and then it just returns it.

This is quite simple (not very useful), but will help us focus on how the sub-dependencies work.

## Second dependency, "dependable" and "dependant"

Then you can create another dependency function (a "dependable") that at the same time declares a dependency of its own (so it is a "dependant" too):

=== "Python 3.10+"

    ```

!!! tip
    Keep in mind that JSON only supports `str` as keys.

    But Pydantic has automatic data conversion.

    This means that, even though your API clients can only send strings as keys, as long as those strings contain pure integers, Pydantic will convert them and validate them.

    And the `dict` you receive as `weights` will actually have `int` keys and `float` values.

## Recap

    end
    operation ->> client: Return response to client
    Note over client,operation: Response is already sent, can't change it anymore
    opt Tasks
        operation -->> tasks: Send background tasks
    end
    opt Raise other exception
        tasks -->> dep: Raise other exception
    end
    Note over dep: After yield
    opt Handle other exception

This will let **FastAPI** know that this parameter is required.

### Required with `None`

You can declare that a parameter can accept `None`, but that it's still required. This would force clients to send a value, even if the value is `None`.

To do that, you can declare that `None` is a valid type but still use `...` as the default:

=== "Python 3.10+"

    ```Python hl_lines="9"

#### A "professional" attack

Of course, the attackers would not try all this by hand, they would write a program to do it, possibly with thousands or millions of tests per second. And would get just one extra correct letter at a time.

But doing that, in some minutes or hours the attackers would have guessed the correct username and password, with the "help" of our application, just using the time taken to answer.