Python异常处理

1.1异常问题举例

例一

>>> i = input('请输入数字')

请输入数字：0
>>> print(i)
0
>>> print(5 / int(i))
Traceback (most recent call last):
  File "", line 1, in 
ZeroDivisionError: division by zero

上述代码的报错是除零的错误，数学上不允许除零

例二

>>> i="QWE"
>>> print(5 / int(i))
Traceback (most recent call last):
  File "", line 1, in 
ValueError: invalid literal for int() with base 10: 'QWE'

i是字符串类型，而且是QWE，不能转换成int类型，所以抛出异常

1.2异常类继承层次

Python中的异常根类是BaseException，结构图如下

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StandardError
      |    +-- BufferError
      |    +-- ArithmeticError
      |    |    +-- FloatingPointError
      |    |    +-- OverflowError
      |    |    +-- ZeroDivisionError
      |    +-- AssertionError
      |    +-- AttributeError
      |    +-- EnvironmentError
      |    |    +-- IOError
      |    |    +-- OSError
      |    |         +-- WindowsError (Windows)
      |    |         +-- VMSError (VMS)
      |    +-- EOFError
      |    +-- ImportError
      |    +-- LookupError
      |    |    +-- IndexError
      |    |    +-- KeyError
      |    +-- MemoryError
      |    +-- NameError
      |    |    +-- UnboundLocalError
      |    +-- ReferenceError
      |    +-- RuntimeError
      |    |    +-- NotImplementedError
      |    +-- SyntaxError
      |    |    +-- IndentationError
      |    |         +-- TabError
      |    +-- SystemError
      |    +-- TypeError
      |    +-- ValueError
      |         +-- UnicodeError
      |              +-- UnicodeDecodeError
      |              +-- UnicodeEncodeError
      |              +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
       +-- ImportWarning
       +-- UnicodeWarning
       +-- BytesWarning

1.3常见异常

1.3.1 AttributeError 异常

访问不存在的成员

例如

>>> class Animal(object):
    pass

>>> a1 = Animal()
>>> a1.run()
Traceback (most recent call last):
  File "", line 1, in 
    a1.run()
AttributeError: 'Animal' object has no attribute 'run'
>>> 
>>> print(a1.age)
Traceback (most recent call last):
  File "", line 1, in 
    print(a1.age)
AttributeError: 'Animal' object has no attribute 'age'

程序中创建了一个类叫Animal，但是并没有run函数和age变量，所以抛出异常

1.3.2 OSError 异常

OSError是操作系统相关异常，例如试图打开一个不存在的文件

>>> f=open("abc.txt")
Traceback (most recent call last):
  File "", line 1, in 
    f=open("abc.txt")
FileNotFoundError: [Errno 2] No such file or directory: 'abc.txt'

1.3.3 IndexError 异常

是访问元素时，下标超出索引最大值或最小值引发异常例如：

>>> code_list = [125, 56, 89, 36]
>>> code_list[4]
Traceback (most recent call last):
  File "", line 1, in 
    code_list[4]
IndexError: list index out of range

1.3.4 KeyError 异常

是试图访问字典里不存在的键引发，例如：

>>> dict1[104]
Traceback (most recent call last):
  File "", line 1, in 
    dict1[104]
NameError: name 'dict1' is not defined

104在字典中不存在，O(∩_∩)O哈哈~那就报错吧

1.3.5 NameError 异常

NameError是指试图使用一个不存在的变量引发的异常，PythonShell中运行实例

>>> value1
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'value1' is not defined
>>> a = value1
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'value1' is not defined
>>> value1 = 10
>>> value1
10

赋值和直接取值都不行，因为没有这个value1变量
只有最后的value1 = 10，才让value1有了值

1.3.6 TypeError 异常

TypeError是试图传入变量类型与要求的不符合时而引发的异常。PythonShell运行实例、

>>> i = '2'
>>> print(5 / i)
Traceback (most recent call last):
  File "", line 1, in 
    print(5 / i)
TypeError: unsupported operand type(s) for /: 'int' and 'str'

i是一个字符类型，而5是整形，不能进行除法运算，因为类型不统一

1.3.7 ValueError 异常

ValueError是由于传入一个无效的参数值而引发的异常，这个异常在前面已经遇到了

>>> del i
>>> i = 'QWE'
>>> print( 5 / int(i))
Traceback (most recent call last):
  File "", line 1, in 
    print( 5 / int(i))
ValueError: invalid literal for int() with base 10: 'QWE'

1.4对于捕获异常的说明

我不会捕获异常，不会使用try和except等等，看也看不懂啊……