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.

# 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.

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).

API 应用调用外部 API 时的流程叫做**回调**。因为外部开发者编写的软件发送请求至您的 API，然后您的 API 要进行回调，并把请求发送至外部 API。

此时，我们需要存档外部 API 的*信息*，比如应该有哪些*路径操作*，返回什么样的请求体，应该返回哪种响应等。

## 使用回调的应用

示例如下。

假设要开发一个创建发票的应用。

发票包括 `id`、`title`（可选）、`customer`、`total` 等属性。

API 的用户 （外部开发者）要在您的 API 内使用 POST 请求创建一条发票记录。

（假设）您的 API 将：

* 把发票发送至外部开发者的消费者
* 归集现金
* 把通知发送至 API 的用户（外部开发者）
    * 通过（从您的 API）发送 POST 请求至外部 API （即**回调**）来完成

## 常规 **FastAPI** 应用

                """
            ),
        ] = None,
        generate_unique_id_function: Annotated[
            Callable[[routing.APIRoute], str],
            Doc(
                """
                Customize the function used to generate unique IDs for the *path
                operations* shown in the generated OpenAPI.

                This is particularly useful when automatically generating clients or

                """
            ),
        ] = True,
        generate_unique_id_function: Annotated[
            Callable[[APIRoute], str],
            Doc(
                """
                Customize the function used to generate unique IDs for the *path
                operations* shown in the generated OpenAPI.

                This is particularly useful when automatically generating clients or