但对于某些高级应用场景，还是需要添加自定义响应头：

```Python hl_lines="14"
{!../../../docs_src/handling_errors/tutorial002.py!}

```

## 安装自定义异常处理器

添加自定义处理器，要使用 [Starlette 的异常工具](https://www.starlette.io/exceptions/)。

假设要触发的自定义异常叫作 `UnicornException`。

且需要 FastAPI 实现全局处理该异常。

此时，可以用 `@app.exception_handler()` 添加自定义异常控制器：

```Python hl_lines="5-7  13-18  24"
{!../../../docs_src/handling_errors/tutorial003.py!}

```

💭 🌐 👈 **"4️⃣0️⃣4️⃣ 🚫 🔎"** ❌ (& 🤣) ❓

## ⚙️ `HTTPException`

📨 🇺🇸🔍 📨 ⏮️ ❌ 👩‍💻 👆 ⚙️ `HTTPException`.

### 🗄 `HTTPException`

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

### 🤚 `HTTPException` 👆 📟

`HTTPException` 😐 🐍 ⚠ ⏮️ 🌖 📊 🔗 🔗.

↩️ ⚫️ 🐍 ⚠, 👆 🚫 `return` ⚫️, 👆 `raise` ⚫️.

### What is "Mounting"

"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.

This is different from using an `APIRouter` as a mounted application is completely independent. The OpenAPI and docs from your main application won't include anything from the mounted application, etc.

from dirty_equals import IsDict
from fastapi.testclient import TestClient

from docs_src.handling_errors.tutorial005 import app

client = TestClient(app)


def test_post_validation_error():
    response = client.post("/items/", json={"title": "towel", "size": "XL"})
    assert response.status_code == 422, response.text
    assert response.json() == IsDict(
        {
            "detail": [
                {

之后，所有处理逻辑都一样。

但因为改变了 `GzipRequest.body`，**FastAPI** 加载请求体时会自动解压。

## 在异常处理器中访问请求体

!!! tip "提示"

    为了解决同样的问题，在 `RequestValidationError` 的自定义处理器使用 `body`  （[处理错误](../tutorial/handling-errors.md#use-the-requestvalidationerror-body){.internal-link target=_blank}）可能会更容易。

    但本例仍然可行，而且本例展示了如何与内部组件进行交互。

同样也可以在异常处理器中访问请求体。

此时要做的只是处理 `try`/`except` 中的请求：

```Python hl_lines="13  15"

## Accessing the request body in an exception handler

!!! tip
    To solve this same problem, it's probably a lot easier to use the `body` in a custom handler for `RequestValidationError` ([Handling Errors](../tutorial/handling-errors.md#use-the-requestvalidationerror-body){.internal-link target=_blank}).

    But this example is still valid and it shows how to interact with the internal components.

You can import it directly form `fastapi`:

```python
from fastapi import WebSocketDisconnect
```

::: fastapi.WebSocketDisconnect

## WebSockets - additional classes

Additional classes for handling WebSockets.

Provided directly by Starlette, but you can import it from `fastapi`:

```python
from fastapi.websockets import WebSocketDisconnect, WebSocketState
```

::: fastapi.websockets.WebSocketDisconnect

👆 👀 👈 👆 💪 ⚙️ 🔗 ⏮️ `yield` & ✔️ `try` 🍫 👈 ✊ ⚠.

⚫️ 5️⃣📆 😋 🤚 `HTTPException` ⚖️ 🎏 🚪 📟, ⏮️ `yield`. ✋️ **⚫️ 🏆 🚫 👷**.

🚪 📟 🔗 ⏮️ `yield` 🛠️ *⏮️* 📨 📨, [⚠ 🐕‍🦺](../handling-errors.md#_4){.internal-link target=_blank} 🔜 ✔️ ⏪ 🏃. 📤 🕳 😽 ⚠ 🚮 👆 🔗 🚪 📟 (⏮️ `yield`).

, 🚥 👆 🤚 `HTTPException` ⏮️ `yield`, 🔢 (⚖️ 🙆 🛃) ⚠ 🐕‍🦺 👈 ✊ `HTTPException`Ⓜ & 📨 🇺🇸🔍 4️⃣0️⃣0️⃣ 📨 🏆 🚫 📤 ✊ 👈 ⚠ 🚫🔜.

from fastapi.testclient import TestClient

from docs_src.handling_errors.tutorial002 import app

client = TestClient(app)


def test_get_item_header():
    response = client.get("/items-header/foo")
    assert response.status_code == 200, response.text
    assert response.json() == {"item": "The Foo Wrestlers"}


def test_get_item_not_found_header():
    response = client.get("/items-header/bar")

## Mounting a **FastAPI** application

"Mounting" means adding a completely "independent" application in a specific path, that then takes care of handling everything under that path, with the _path operations_ declared in that sub-application.

### Top-level application

First, create the main, top-level, **FastAPI** application, and its *path operations*: