```Python hl_lines="7"
{!../../../docs_src/query_params_str_validations/tutorial006b.py!}
```

!!! info
    🚥 👆 🚫 👀 👈 `...` ⏭: ⚫️ 🎁 👁 💲, ⚫️ <a href="https://docs.python.org/3/library/constants.html#Ellipsis" class="external-link" target="_blank">🍕 🐍 &amp; 🤙 "❕"</a>.

    ⚫️ ⚙️ Pydantic &amp; FastAPI 🎯 📣 👈 💲 ✔.

👉 🔜 ➡️ **FastAPI** 💭 👈 👉 🔢 ✔.

### ✔ ⏮️ `None`

It's relatively tightly coupled with relational databases (like MySQL or PostgreSQL), so, having a NoSQL database (like Couchbase, MongoDB, Cassandra, etc) as the main store engine is not very easy.

It was created to generate the HTML in the backend, not to create APIs used by a modern frontend (like React, Vue.js and Angular) or by other systems (like <abbr title="Internet of Things">IoT</abbr> devices) communicating with it.

🖼 👆 💪 ✔️ 📁 `main.py` ⏮️:

```Python hl_lines="3"
import os

name = os.getenv("MY_NAME", "World")
print(f"Hello {name} from Python")
```

!!! tip
    🥈 ❌ <a href="https://docs.python.org/3.8/library/os.html#os.getenv" class="external-link" target="_blank">`os.getenv()`</a> 🔢 💲 📨.

    🚥 🚫 🚚, ⚫️ `None` 🔢, 📥 👥 🚚 `"World"` 🔢 💲 ⚙️.

⤴️ 👆 💪 🤙 👈 🐍 📋:

<div class="termy">

```console

```Python hl_lines="7  21  23"
{!../../../docs_src/custom_response/tutorial004.py!}
```

In diesem Beispiel generiert die Funktion `generate_html_response()` bereits eine `Response` und gibt sie zurück, anstatt das HTML in einem `str` zurückzugeben.

Indem Sie das Ergebnis des Aufrufs von `generate_html_response()` zurückgeben, geben Sie bereits eine `Response` zurück, die das Standardverhalten von **FastAPI** überschreibt.

Поля `example` как такового не существует в стандартах **JSON Schema**. В последних версиях JSON-схемы определено поле <a href="https://json-schema.org/draft/2019-09/json-schema-validation.html#rfc.section.9.5" class="external-link" target="_blank">`examples`</a>, но OpenAPI 3.0.3 основан на более старой версии JSON-схемы, которая не имела поля `examples`.

```Python hl_lines="7  21  23"
{!../../../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.

它看起来像这样：

<img src="/img/tutorial/sql-databases/image02.png">

您还可以使用[SQLite Viewer](https://inloop.github.io/sqlite-viewer/)或[ExtendsClass](https://extendsclass.com/sqlite-browser.html)等在线 SQLite 浏览器。

## 中间件替代数据库会话

如果你不能使用依赖项`yield`——例如，如果你没有使用**Python 3.7**并且不能安装上面提到的**Python 3.6**的“backports” ——你可以在类似的“中间件”中设置会话方法。

“中间件”基本功能是一个为每个请求执行的函数在请求之前进行执行相应的代码，以及在请求执行之后执行相应的代码。

### 创建中间件

```Python hl_lines="1  6-9"
{!../../../docs_src/path_params/tutorial005.py!}
```

!!! info
    <a href="https://docs.python.org/3/library/enum.html" class="external-link" target="_blank">Enumerationen (oder kurz Enums)</a> gibt es in Python seit Version 3.4.

!!! tip "Tipp"

    # Ref: JSON Schema Validation 2020-12: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-vocabularies-for-semantic-c
    # Vocabularies for Semantic Content With "format"
    format: Optional[str] = None
    # Ref: JSON Schema Validation 2020-12: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-a-vocabulary-for-the-conten
    # A Vocabulary for the Contents of String-Encoded Data

!!! note "Technische Details"
    Jede Funktion, die dekoriert werden kann mit:

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager" class="external-link" target="_blank">`@contextlib.contextmanager`</a> oder
    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>