Python异常处理机制详解

发布时间: 2025-03-24 13:22

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# Python异常处理机制详解

本文将详细介绍Python中的异常处理机制,帮助你更好地理解和处理程序中可能出现的错误情况。

## 异常基础

### 什么是异常

异常是程序运行时发生的错误。当Python检测到一个错误时,它会创建一个异常对象。如果异常对象未被处理,程序将会终止并显示一个错误信息。

### 常见的异常类型

```python
# TypeError - 类型错误
print("123" + 456)    # TypeError: can only concatenate str (not "int") to str

# ValueError - 值错误
int("abc")            # ValueError: invalid literal for int() with base 10: 'abc'

# NameError - 名称错误
print(undefined_var)   # NameError: name 'undefined_var' is not defined

# IndexError - 索引错误
list = [1, 2, 3]
print(list[3])         # IndexError: list index out of range

# KeyError - 键错误
dict = {"a": 1}
print(dict["b"])       # KeyError: 'b'

# FileNotFoundError - 文件未找到错误
open("nonexistent.txt")  # FileNotFoundError: [Errno 2] No such file or directory

# ZeroDivisionError - 除零错误
print(10 / 0)           # ZeroDivisionError: division by zero
```

## 异常处理

### try-except语句

```python
# 基本异常处理
try:
    num = int(input("请输入一个数字: "))
    result = 10 / num
    print(f"结果是: {result}")
except ValueError:
    print("请输入有效的数字!")
except ZeroDivisionError:
    print("除数不能为零!")

# 处理多个异常
try:
    file = open("test.txt")
    content = file.read()
    number = int(content)
except FileNotFoundError:
    print("文件不存在")
except ValueError:
    print("文件内容不是有效的数字")
finally:
    file.close()  # 确保文件被关闭
```

### else和finally子句

```python
try:
    num = int(input("请输入一个正数: "))
    if num <= 0:
        raise ValueError("输入必须是正数")
except ValueError as e:
    print(f"错误: {e}")
else:
    # 当try块中没有异常发生时执行
    print(f"你输入的数字是: {num}")
finally:
    # 无论是否发生异常都会执行
    print("程序执行结束")
```

### 异常的传播

```python
def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("除数不能为零!")
        raise  # 重新抛出异常

try:
    divide(10, 0)
except ZeroDivisionError:
    print("捕获到除零错误")
```

## 自定义异常

### 创建自定义异常类

```python
class CustomError(Exception):
    """自定义异常类"""
    def __init__(self, message):
        self.message = message
        super().__init__(self.message)

# 使用自定义异常
def validate_age(age):
    if age < 0:
        raise CustomError("年龄不能为负数")
    if age > 150:
        raise CustomError("年龄不能超过150岁")
    return age

try:
    age = validate_age(-5)
except CustomError as e:
    print(f"错误: {e}")
```

## 实际应用示例

### 1. 文件处理

```python
class FileHandler:
    def __init__(self, filename):
        self.filename = filename
        self.file = None
    
    def open_file(self):
        try:
            self.file = open(self.filename, 'r')
        except FileNotFoundError:
            print(f"文件 {self.filename} 不存在")
            return False
        except PermissionError:
            print(f"没有权限访问文件 {self.filename}")
            return False
        return True
    
    def read_integers(self):
        if not self.open_file():
            return []
        
        numbers = []
        try:
            for line in self.file:
                try:
                    num = int(line.strip())
                    numbers.append(num)
                except ValueError:
                    print(f"忽略无效的行: {line.strip()}")
        finally:
            self.file.close()
        
        return numbers

# 使用示例
handler = FileHandler("numbers.txt")
result = handler.read_integers()
print(f"读取的数字: {result}")
```

### 2. 网络请求处理

```python
import requests
from time import sleep

class NetworkError(Exception):
    """网络错误异常"""
    pass

class APIClient:
    def __init__(self, base_url):
        self.base_url = base_url
    
    def get_data(self, endpoint, max_retries=3):
        for attempt in range(max_retries):
            try:
                response = requests.get(f"{self.base_url}{endpoint}")
                response.raise_for_status()  # 检查HTTP错误
                return response.json()
            except requests.RequestException as e:
                if attempt == max_retries - 1:
                    raise NetworkError(f"请求失败: {e}")
                print(f"重试请求 ({attempt + 1}/{max_retries})")
                sleep(1)  # 等待1秒后重试

# 使用示例
client = APIClient("https://api.example.com")
try:
    data = client.get_data("/users")
    print("获取的数据:", data)
except NetworkError as e:
    print(f"错误: {e}")
```

### 3. 数据验证

```python
class ValidationError(Exception):
    """数据验证错误"""
    pass

class User:
    def __init__(self, username, email, age):
        self.username = self.validate_username(username)
        self.email = self.validate_email(email)
        self.age = self.validate_age(age)
    
    @staticmethod
    def validate_username(username):
        if not username:
            raise ValidationError("用户名不能为空")
        if len(username) < 3:
            raise ValidationError("用户名长度必须大于3个字符")
        return username
    
    @staticmethod
    def validate_email(email):
        if not "@" in email:
            raise ValidationError("无效的邮箱地址")
        return email
    
    @staticmethod
    def validate_age(age):
        try:
            age = int(age)
            if age < 0 or age > 150:
                raise ValidationError("年龄必须在0-150之间")
            return age
        except ValueError:
            raise ValidationError("年龄必须是数字")

# 使用示例
try:
    user = User("Al", "invalid_email", "not_a_number")
except ValidationError as e:
    print(f"验证错误: {e}")
```

## 最佳实践

1. **具体异常优于普通异常**
```python
# 不好的做法
try:
    # 一些操作
    pass
except Exception as e:
    print(f"发生错误: {e}")

# 好的做法
try:
    # 一些操作
    pass
except ValueError:
    print("值错误")
except TypeError:
    print("类型错误")
```

2. **避免空的except子句**
```python
# 不好的做法
try:
    num = int("abc")
except:
    pass  # 忽略所有错误

# 好的做法
try:
    num = int("abc")
except ValueError:
    print("无法转换为整数")
```

3. **正确使用finally**
```python
# 不好的做法
file = open("test.txt")
try:
    data = file.read()
finally:
    file.close()

# 好的做法
with open("test.txt") as file:
    data = file.read()
# 文件会自动关闭
```

4. **合理使用自定义异常**
```python
# 不好的做法
def process_age(age):
    if age < 0:
        raise Exception("年龄不能为负数")

# 好的做法
class AgeError(Exception):
    """年龄相关的错误"""
    pass

def process_age(age):
    if age < 0:
        raise AgeError("年龄不能为负数")
```

通过本文的学习,你应该已经掌握了Python异常处理的基本概念和使用方法。异常处理是编写健壮程序的重要部分,合理使用异常处理机制可以让你的程序更加可靠和易于维护。继续练习和探索,你会发现更多异常处理的应用场景！

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