Python 基础系列 11 - Python 异常处理 Exception Handling_python exception handler

引言

本文主要梳理一下 Python 异常 Exception 处理 ，更多 Python 基础系列，请参考 Python 基础系列大纲

内容提要：

• 异常 Exception 对象
• 异常处理 Exception Handling
  关键字机制支持异常处理
  try -> except -> else -> finally
  分组异常处理
  泛型异常 Exception Handling
  raise 抛出异常
• 异常角色说明

异常 Exception 对象

下面例子是日常 Python 代码中遇到常见的异常：

什么是异常对象？
异常 Exception 对象是管理运行时抛出的错误。

异常对象的用途？
帮助处理类似下面状况：

• 文件不存在
• 除 0
• index 越界

异常发生的现象：
Python 代码会中断并打印栈跟踪信息，用来解释哪个文件，哪行代码出现异常。

为什么处理异常？

• 阻止代码崩塌
• 提高代码健壮性，防止恶意地中断程序
• 提高程序的可用性，稳定性

不同异常类型对应的异常对象：

异常处理 Exception Handling

关键字机制支持异常处理

• Try --> 捕获 Catch 异常 exceptions
• except, else, 和 finally --> 从异常中恢复
  处理异常
  报告异常
• raise --> 抛出异常
  从用户代码中显示地抛出异常

try -> except -> else -> finally

try: 捕获异常
except: 处理异常
else: 没有异常时执行
finally: 无论是否异常都会最后执行

实践

n = input('Nominator: ') 
d = input('Denominator: ') 
try: 
    devision = float(n) / float(d) 
except ZeroDivisionError as e: 
    print ('Error handle-Exception: {} \n Type: {} \ \n Tuple of Messages: {} ' .format(e, type(e), e.args)) 
else:
    print('Perform_if_try_is_successfully')
finally: 
    print ('Always perform action: Trying to divide…') 
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没有异常时, 执行 else->finally

# output:
Nominator: 6
Denominator: 2
Perform_if_try_is_successfully
Always perform action: Trying to divide…
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异常发生, 执行 exception->finally
e.args: 异常的额外一些信息

# output:
Nominator: 6
Denominator: 0
Error handle-Exception: float division by zero 
 Type: <class 'ZeroDivisionError'> \ 
 Tuple of Messages: ('float division by zero',) 
Always perform action: Trying to divide…
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异常发生, 但是异常类型不匹配, 执行 finally->并抛出异常
注意因为没有处理异常，会打印栈跟踪信息。

# output:
Nominator: aa
Denominator: 3
Always perform action: Trying to divide…

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-282-3eb1b8a9856e> in <module>
      2 d = input('Denominator: ')
      3 try:
----> 4     devision = float(n) / float(d)
      5 except ZeroDivisionError as e:
      6     print ('Error handle-Exception: {} \n Type: {} \ \n Tuple of Messages: {} ' .format(e, type(e), e.args))

ValueError: could not convert string to float: 'aa'
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分组异常处理

n = input('Nominator: ') 
d = input('Denominator: ') 
try: 
    devision = float(n) / float(d) 
except (ValueError, ZeroDivisionError) as e: 
    print ('Error handle-Exception: {} \n Type: {} \ \n Tuple of Messages: {} ' .format(e, type(e), e.args)) 
else:
    print('Perform_if_try_is_successfully')
finally: 
    print ('Always perform action: Trying to divide…') 
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泛型异常 Exception Handling

避免泛型异常处理，最好指明异常类型。

n = input('Nominator: ') 
d = input('Denominator: ') 
try: 
    devision = float(n) / float(d) 
except Exception as e: 
    print ('Error handle-Exception: {} \n Type: {} \ \n Tuple of Messages: {} ' .format(e, type(e), e.args)) 
else:
    print('Perform_if_try_is_successfully')
finally: 
    print ('Always perform action: Trying to divide…') 
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raise 抛出异常

用于明确地抛出内建的或用户自定义的异常。

raise Exception_type ([value])

例如： raise RuntimeError (“Unrecoverable Error”)

def divide_operation(nominator, denominator):
    if denominator == 0:
        raise ZeroDivisionError ("Which grade are you in")
    else:
        return float(nominator) / float(denominator)
                                 
print(divide_operation(5, 2))
divide_operation(5, 0)  
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# output:
2.5

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-291-7c2394d04a90> in <module>
      6 
      7 print(divide_operation(5, 2))
----> 8 divide_operation(5, 0)

<ipython-input-291-7c2394d04a90> in divide_operation(nominator, denominator)
      1 def divide_operation(nominator, denominator):
      2     if denominator == 0:
----> 3         raise ZeroDivisionError ("Which grade are you in")
      4     else:
      5         return float(nominator) / float(denominator)

ZeroDivisionError: Which grade are you in
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异常角色说明

练习作业：