* The **file** that can be **executed** by the operating system, for example: `python`, `python.exe` or `uvicorn`.
* A particular program while it is **running** on the operating system, using the CPU, and storing things on memory. This is also called a **process**.

### What is a Process

The word **process** is normally used in a more specific way, only referring to the thing that is running in the operating system (like in the last point above):

"docs_src/dependencies/tutorial010.py" = ["F821"]
"docs_src/custom_response/tutorial007.py" = ["B007"]
"docs_src/dataclasses/tutorial003.py" = ["I001"]
"docs_src/path_operation_advanced_configuration/tutorial007.py" = ["B904"]
"docs_src/path_operation_advanced_configuration/tutorial007_pv1.py" = ["B904"]
"docs_src/custom_request_and_route/tutorial002.py" = ["B904"]
"docs_src/dependencies/tutorial008_an.py" = ["F821"]

An <a href="https://en.wikipedia.org/wiki/Environment_variable" class="external-link" target="_blank">environment variable</a> (also known as "env var") is a variable that lives outside of the Python code, in the operating system, and could be read by your Python code (or by other programs as well).

You can create and use environment variables in the shell, without needing Python:

=== "Linux, macOS, Windows Bash"

    <div class="termy">

    🚥 👆 🚫 "🕴" 🗄, 👆 🎲 🚫 💪 👉.

👆 💪 ⚒ 🗄 `operationId` ⚙️ 👆 *➡ 🛠️* ⏮️ 🔢 `operation_id`.

👆 🔜 ✔️ ⚒ 💭 👈 ⚫️ 😍 🔠 🛠️.

```Python hl_lines="6"
{!../../../docs_src/path_operation_advanced_configuration/tutorial001.py!}
```

### ⚙️ *➡ 🛠️ 🔢* 📛 {

🚥 👆 💚 ⚙️ 👆 🔗' 🔢 📛 `operationId`Ⓜ, 👆 💪 🔁 🤭 🌐 👫 & 🔐 🔠 *➡ 🛠️* `operation_id` ⚙️ 👫 `APIRoute.name`.

👆 🔜 ⚫️ ⏮️ ❎ 🌐 👆 *➡ 🛠️*.

```

...that's because the client generator uses the OpenAPI internal **operation ID** for each *path operation*.

OpenAPI requires that each operation ID is unique across all the *path operations*, so FastAPI uses the **function name**, the **path**, and the **HTTP method/operation** to generate that operation ID, because that way it can make sure that the operation IDs are unique.

But I'll show you how to improve that next. 🤓

But here's the key point.

The security and dependency injection stuff is written once.

And you can make it as complex as you want. And still, have it written only once, in a single place. With all the flexibility.

But you can have thousands of endpoints (*path operations*) using the same security system.

    It is the "**path operation decorator**".

You can also use the other operations:

* `@app.post()`
* `@app.put()`
* `@app.delete()`

And the more exotic ones:

* `@app.options()`
* `@app.head()`
* `@app.patch()`
* `@app.trace()`

!!! tip
    You are free to use each operation (HTTP method) as you wish.

## Order matters

When creating *path operations*, you can find situations where you have a fixed path.

Like `/users/me`, let's say that it's to get data about the current user.

And then you can also have a path `/users/{user_id}` to get data about a specific user by some user ID.

│   ├── main.py
│   └── test_main.py
```

Let's say that now the file `main.py` with your **FastAPI** app has some other **path operations**.

It has a `GET` operation that could return an error.

It has a `POST` operation that could return several errors.

Both *path operations* require an `X-Token` header.

=== "Python 3.10+"

    ```Python
    {!> ../../../docs_src/app_testing/app_b_an_py310/main.py!}

* データ処理:
    * たとえば、時間のかかる処理を必要とするファイル受信時には、「受信済み」(HTTP 202) のレスポンスを返し、バックグラウンドで処理できます。

## `BackgroundTasks` の使用

まず初めに、`BackgroundTasks` をインポートし、` BackgroundTasks` の型宣言と共に、*path operation 関数* のパラメーターを定義します:

```Python hl_lines="1  13"
{!../../../docs_src/background_tasks/tutorial001.py!}
```

**FastAPI** は、`BackgroundTasks` 型のオブジェクトを作成し、そのパラメーターに渡します。

## タスク関数の作成