# 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` ⚫️.

    validations:
      required: true

  - type: textarea
    id: error-handling-proposal
    attributes:
      label: Does this affect error handling?
      description: If so, how does this differ from previous error handling proposals?
      placeholder: |
       Yes or No

       If yes, 
        1.how does this differ from previous error handling proposals?

    validations:
      required: true

  - type: textarea

// one of the CMP instructions that require special handling.
func IsLoong64CMP(op obj.As) bool {
	switch op {
	case loong64.ACMPEQF, loong64.ACMPEQD, loong64.ACMPGEF, loong64.ACMPGED,
		loong64.ACMPGTF, loong64.ACMPGTD:
		return true
	}
	return false
}

// IsLoong64MUL reports whether the op (as defined by an loong64.A* constant) is
// one of the MUL/DIV/REM instructions that require special handling.
func IsLoong64MUL(op obj.As) bool {

	tw := tar.NewWriter(&buf)
	var files = []struct {
		Name, Body string
	}{
		{"readme.txt", "This archive contains some text files."},
		{"gopher.txt", "Gopher names:\nGeorge\nGeoffrey\nGonzo"},
		{"todo.txt", "Get animal handling license."},
	}
	for _, file := range files {
		hdr := &tar.Header{
			Name: file.Name,
			Mode: 0600,
			Size: int64(len(file.Body)),
		}
		if err := tw.WriteHeader(hdr); err != nil {
			log.Fatal(err)

## 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.

// one of the CMP instructions that require special handling.
func IsPPC64CMP(op obj.As) bool {
	switch op {
	case ppc64.ACMP, ppc64.ACMPU, ppc64.ACMPW, ppc64.ACMPWU, ppc64.AFCMPU:
		return true
	}
	return false
}

// IsPPC64NEG reports whether the op (as defined by an ppc64.A* constant) is
// one of the NEG-like instructions that require special handling.
func IsPPC64NEG(op obj.As) bool {
	switch op {

  <modules>
    <module>child</module>
  </modules>

  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <!-- this checks handling of a plugin which is implicitly bound to the lifecycle -->
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-resources-plugin</artifactId>
          <version>0.1-stub-SNAPSHOT</version>

// one of the CMP instructions that require special handling.
func IsMIPSCMP(op obj.As) bool {
	switch op {
	case mips.ACMPEQF, mips.ACMPEQD, mips.ACMPGEF, mips.ACMPGED,
		mips.ACMPGTF, mips.ACMPGTD:
		return true
	}
	return false
}

// IsMIPSMUL reports whether the op (as defined by an mips.A* constant) is
// one of the MUL/DIV/REM/MADD/MSUB instructions that require special handling.
func IsMIPSMUL(op obj.As) bool {
	switch op {