```

FastAPI will use this return type to:

* **Validate** the returned data.
    * If the data is invalid (e.g. you are missing a field), it means that *your* app code is broken, not returning what it should, and it will return a server error instead of returning incorrect data. This way you and your clients can be certain that they will receive the data and the data shape expected.

You will have inline errors for the data that you send:

<img src="/img/tutorial/generate-clients/image04.png">

The response object will also have autocompletion:

<img src="/img/tutorial/generate-clients/image05.png">

## FastAPI App with Tags

In many cases your FastAPI app will be bigger, and you will probably use tags to separate different groups of *path operations*.

<div class="termy">

```console
$ python -m venv env
```

</div>

That will create a directory `./env/` with the Python binaries, and then you will be able to install packages for that local environment.

### Activate the environment

Activate the new environment with:

=== "Linux, macOS"

    <div class="termy">

```JSON
{
    "message": "Hello World",
    "root_path": "/api/v1"
}
```

So, it won't expect to be accessed at `http://127.0.0.1:8000/api/v1/app`.

You will see the JSON response as:

```JSON
{"message": "Hello World"}
```

### Interactive API docs

Now go to <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>.

🚥 👆 ❎ 🏎 ✳, 👆 💪 ❎ 🛠️ ⏮️:

<div class="termy">

```console
$ typer --install-completion

zsh completion installed in /home/user/.bashrc.
Completion will take effect once you restart the terminal.
```

</div>

### 📱 &amp; 🩺 🎏 🕰

🚥 👆 🏃 🖼 ⏮️, ✅:

<div class="termy">

```console
$ uvicorn tutorial001:app --reload

* OS上で**実行**している特定のプログラム
    * これはファイルやコードを指すのではなく、OSによって**実行**され、管理されているものを指します。
* どんなプログラムやコードも、それが**実行されているときにだけ機能**します。つまり、**プロセスとして実行されているときだけ**です。
* プロセスは、ユーザーにあるいはOSによって、 **終了**（あるいは "kill"）させることができます。その時点で、プロセスは実行/実行されることを停止し、それ以降は**何もできなくなります**。
* コンピュータで実行されている各アプリケーションは、実行中のプログラムや各ウィンドウなど、その背後にいくつかのプロセスを持っています。そして通常、コンピュータが起動している間、**多くのプロセスが**同時に実行されています。
* **同じプログラム**の**複数のプロセス**が同時に実行されていることがあります。

### Server Memory

For example, if your code loads a Machine Learning model with **1 GB in size**, when you run one process with your API, it will consume at least 1 GB of RAM. And if you start **4 processes** (4 workers), each will consume 1 GB of RAM. So in total, your API will consume **4 GB of RAM**.

So, the prefix in this case is `/items`.

We can also add a list of `tags` and extra `responses` that will be applied to all the *path operations* included in this router.

And we can add a list of `dependencies` that will be added to all the *path operations* in the router and will be executed/solved for each request made to them.

!!! tip

Later you will see how **powerful** it can be.

!!! tip
    The fact that this is **standard Python** means that you will still get the **best possible developer experience** in your editor, with the tools you use to analyze and refactor your code, etc. ✨

    And also that your code will be very compatible with many other Python tools and libraries. 🚀