Sehen wir uns das alles anhand eines Beispiels an.

Stellen Sie sich vor, Sie entwickeln eine Anwendung, mit der Sie Rechnungen erstellen können.

Diese Rechnungen haben eine `id`, einen optionalen `title`, einen `customer` (Kunde) und ein `total` (Gesamtsumme).

Der Benutzer Ihrer API (ein externer Entwickler) erstellt mit einem POST-Request eine Rechnung in Ihrer API.

Dann wird Ihre API (beispielsweise):

👉 💼, 👆 💪 💚 📄 ❔ 👈 🔢 🛠️ *🔜* 👀 💖. ⚫️❔ *➡ 🛠️* ⚫️ 🔜 ✔️, ⚫️❔ 💪 ⚫️ 🔜 ⌛, ⚫️❔ 📨 ⚫️ 🔜 📨, ♒️.

## 📱 ⏮️ ⏲

➡️ 👀 🌐 👉 ⏮️ 🖼.

🌈 👆 🛠️ 📱 👈 ✔ 🏗 🧾.

👉 🧾 🔜 ✔️ `id`, `title` (📦), `customer`, & `total`.

👩‍💻 👆 🛠️ (🔢 👩‍💻) 🔜 ✍ 🧾 👆 🛠️ ⏮️ 🏤 📨.

⤴️ 👆 🛠️ 🔜 (➡️ 🌈):

* 📨 🧾 🕴 🔢 👩‍💻.
* 📈 💸.
* 📨 📨 🔙 🛠️ 👩‍💻 (🔢 👩‍💻).

### Custom Generate Unique ID Function

FastAPI uses a **unique ID** for each *path operation*, it is used for the **operation ID** and also for the names of any needed custom models, for requests or responses.

You can customize that function. It takes an `APIRoute` and outputs a string.

For example, here it is using the first tag (you will probably have only one tag) and the *path operation* name (the function name).

        )


class OAuth2(SecurityBase):
    """
    This is the base class for OAuth2 authentication, an instance of it would be used
    as a dependency. All other OAuth2 classes inherit from it and customize it for
    each OAuth2 flow.

    You normally would not create a new class inheriting from it but use one of the
    existing subclasses, and maybe compose them if you want to support multiple flows.

To add metadata to your OpenAPI schema, including a license, version, contact, etc, read the docs for [Tutorial - Metadata and Docs URLs](../tutorial/metadata.md){.internal-link target=_blank}.

## OpenAPI Custom URL

To customize the OpenAPI URL (or remove it), read the docs for [Tutorial - Metadata and Docs URLs](../tutorial/metadata.md#openapi-url){.internal-link target=_blank}.

## OpenAPI Docs URLs

from typing import Union

from fastapi import APIRouter, FastAPI
from pydantic import BaseModel, HttpUrl

app = FastAPI()


class Invoice(BaseModel):
    id: str
    title: Union[str, None] = None
    customer: str
    total: float


class InvoiceEvent(BaseModel):
    description: str
    paid: bool


class InvoiceEventReceived(BaseModel):
    ok: bool


invoices_callback_router = APIRouter()

# Metadata and Docs URLs

You can customize several metadata configurations in your **FastAPI** application.

## Metadata for API

You can set the following fields that are used in the OpenAPI specification and the automatic API docs UIs:

| Parameter | Type | Description |
|------------|------|-------------|
| `title` | `str` | The title of the API. |

    * PR [#278](https://github.com/tiangolo/fastapi/pull/278).

## 0.26.0

* Separate error handling for validation errors.
    * This will allow developers to customize the exception handlers.
    * Document better how to handle exceptions and use error handlers.

                            # TODO: remove when deprecating Pydantic v1
                            {"title": "Title", "type": "string"}
                        ),
                        "customer": {"title": "Customer", "type": "string"},
                        "total": {"title": "Total", "type": "number"},
                    },
                },
                "InvoiceEvent": {

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

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).