# Handling Errors

There are many situations in which you need to notify an error to a client that is using your API.

This client could be a browser with a frontend, a code from someone else, an IoT device, etc.

You could need to tell the client that:

* The client doesn't have enough privileges for that operation.
* The client doesn't have access to that resource.
* The item the client was trying to access doesn't exist.
* etc.

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` ⚫️.

## 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.

### 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.

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 📨 🏆 🚫 📤 ✊ 👈 ⚠ 🚫🔜.

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"

## 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*:

⏮️ 👈, 🌐 🏭 ⚛ 🎏.

✋️ ↩️ 👆 🔀 `GzipRequest.body`, 📨 💪 🔜 🔁 🗜 🕐❔ ⚫️ 📐 **FastAPI** 🕐❔ 💪.

## 🔐 📨 💪 ⚠ 🐕‍🦺

!!! tip
    ❎ 👉 🎏 ⚠, ⚫️ 🎲 📚 ⏩ ⚙️ `body` 🛃 🐕‍🦺 `RequestValidationError` ([🚚 ❌](../tutorial/handling-errors.md#requestvalidationerror){.internal-link target=_blank}).

    ✋️ 👉 🖼 ☑ & ⚫️ 🎦 ❔ 🔗 ⏮️ 🔗 🦲.

👥 💪 ⚙️ 👉 🎏 🎯 🔐 📨 💪 ⚠ 🐕‍🦺.

🌐 👥 💪 🍵 📨 🔘 `try`/`except` 🍫:

```Python hl_lines="13  15"