So, you will be able to see it all in the interactive API docs.

## Connect and disconnect

* Create your `FastAPI` application.
* Create event handlers to connect and disconnect from the database.

```Python hl_lines="42  45-47  50-52"
{!../../../docs_src/async_sql_databases/tutorial001.py!}
```

## Read notes

Create the *path operation function* to read notes:

        * [Override the default exception handlers](https://fastapi.tiangolo.com/tutorial/handling-errors/#override-the-default-exception-handlers).
        * [Re-use **FastAPI's** exception handlers](https://fastapi.tiangolo.com/tutorial/handling-errors/#re-use-fastapis-exception-handlers).
    * PR [#273](https://github.com/tiangolo/fastapi/pull/273).

      advanced/conditional-openapi.md: how-to/conditional-openapi.md
      advanced/extending-openapi.md: how-to/extending-openapi.md
  mkdocstrings:
    handlers:
      python:
        options:
          extensions:
          - griffe_typingdoc
          show_root_heading: true
          show_if_no_docstring: true
          preload_modules:
          - httpx

    You can probably skip this part.

There's an alternative way to define this logic to be executed during *startup* and during *shutdown*.

您看到可以使用带有 `yield` 的依赖项，并且具有捕获异常的 `try` 块。

在 `yield` 后抛出 `HTTPException` 或类似的异常是很诱人的，但是**这不起作用**。

带有`yield`的依赖中的退出代码在响应发送之后执行，因此[异常处理程序](../handling-errors.md#install-custom-exception-handlers){.internal-link target=_blank}已经运行过。没有任何东西可以捕获退出代码（在`yield`之后）中的依赖抛出的异常。

所以，如果在`yield`之后抛出`HTTPException`，默认（或任何自定义）异常处理程序捕获`HTTPException`并返回HTTP 400响应的机制将不再能够捕获该异常。

这就是允许在依赖中设置的任何东西（例如数据库会话(DB session)）可以被后台任务使用的原因。

{% endfor %}
{% endif %}

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