Python学习笔记——异常

1. 一个例子

file_name = input('请输入要打开的文件名：')
f = open(file_name)
print('文件的内容是：')
for each_line in f:
    print(each_line)

# 由于未输入文件后缀而报错

请输入要打开的文件名：girl_1

---------------------------------------------------------------------------

FileNotFoundError                         Traceback (most recent call last)

<ipython-input-1-92806aeac0ba> in <module>()
      1 file_name = input('请输入要打开的文件名：')
----> 2 f = open(file_name)
      3 print('文件的内容是：')
      4 for each_line in f:
      5     print(each_line)

FileNotFoundError: [Errno 2] No such file or directory: 'girl_1'

2. 异常种类

以下是 Python 内置异常类的层次结构：

'''
BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
      +-- StopIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning
'''

3. AssertionError

# python assert断言是声明其布尔值必须为真的判定，如果发生异常就说明表达示为假。
my_list = ['小甲鱼是帅哥']
assert len(my_list) > 0
my_list.pop()

'小甲鱼是帅哥'

assert len(my_list) > 0

---------------------------------------------------------------------------

AssertionError                            Traceback (most recent call last)

<ipython-input-9-c183f6b56427> in <module>()
----> 1 assert len(my_list) > 0

AssertionError: 

4. AttributeError

# 访问未知的属性或方法
my_list.fishc

---------------------------------------------------------------------------

AttributeError                            Traceback (most recent call last)

<ipython-input-10-6cd380373ff6> in <module>()
----> 1 my_list.fishc

AttributeError: 'list' object has no attribute 'fishc'

5. IndexError

# 索引不存在
my_list = [1,2,3]
my_list[3]

---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-12-630619d25352> in <module>()
      1 # 索引不存在
      2 my_list = [1,2,3]
----> 3 my_list[3]

IndexError: list index out of range

6.KeyError

# 关键字不存在
my_dict = {'one':1,'two':2,'three':3}
print(my_dict['one'])
print(my_dict['four'])

1

---------------------------------------------------------------------------

KeyError                                  Traceback (most recent call last)

<ipython-input-17-5fd29812ea8d> in <module>()
      2 my_dict = {'one':1,'two':2,'three':3}
      3 print(my_dict['one'])
----> 4 print(my_dict['four'])

KeyError: 'four'

7.NameError

# 访问一个不存在的变量
print(fishc)

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-16-cfe17886c343> in <module>()
      1 # 访问一个不存在的变量
----> 2 print(fishc)

NameError: name 'fishc' is not defined

8. OSError

# 如第一个例子FileNotFoundError就是它的子类

9. SyntaxError

# 语法错误
print 'I love you'

  File "<ipython-input-18-3552d36a503f>", line 2
    print 'I love you'
                     ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('I love you')?

10. TypeError

# 不同类型间的无效操作
print(1 + '1')

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-19-27cba25553e1> in <module>()
      1 # 不同类型间的无效操作
----> 2 print(1 + '1')

TypeError: unsupported operand type(s) for +: 'int' and 'str'

11. ZeroDivisionError

# 除数为0
print(5/0)

---------------------------------------------------------------------------

ZeroDivisionError                         Traceback (most recent call last)

<ipython-input-21-fb41e7d29eb0> in <module>()
      1 # 除数为0
----> 2 print(5/0)

ZeroDivisionError: division by zero

12. 异常检测——try-except

# 语法
'''
try:
    检测范围
except Exception[as reason]:
    出现异常（Exception）后的处理代码
'''

报错

ff = open('我为什么是一个文件.txt')
print(ff.read())
ff.close()

---------------------------------------------------------------------------

FileNotFoundError                         Traceback (most recent call last)

<ipython-input-22-a3241d9cac63> in <module>()
----> 1 ff = open('我为什么是一个文件.txt')
      2 print(ff.read())
      3 ff.close()

FileNotFoundError: [Errno 2] No such file or directory: '我为什么是一个文件.txt'

捕获异常并给出提示

try:
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except OSError:
    print('文件出错啦！！')

文件出错啦！！

捕获异常并给出错误原因

try:
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except OSError as reason:
    print('文件出错啦！！')
    print('错误的原因是：' + str(reason))

文件出错啦！！
错误的原因是：[Errno 2] No such file or directory: '我为什么是一个文件.txt'

有多个异常

# 只捕获第一个异常，因为程序运行到第一个异常就会停止运行
try:
    sum = 1 + '1'
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except OSError as reason:
    print('文件出错啦！！')
    print('错误的原因是：' + str(reason))
except TypeError as reason:
    print('类型出错啦！！')
    print('错误的原因是：' + str(reason))

类型出错啦！！
错误的原因是：unsupported operand type(s) for +: 'int' and 'str'

# 若未捕获相应类型的异常，仍会报错
try:
    int('abc')
    sum = 1 + '1'
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except OSError as reason:
    print('文件出错啦！！')
    print('错误的原因是：' + str(reason))
except TypeError as reason:
    print('类型出错啦！！')
    print('错误的原因是：' + str(reason))

---------------------------------------------------------------------------

ValueError                                Traceback (most recent call last)

<ipython-input-28-23ae102eaf21> in <module>()
      1 # 若未捕获相应类型的异常，仍会报错
      2 try:
----> 3     int('abc')
      4     sum = 1 + '1'
      5     ff = open('我为什么是一个文件.txt')

ValueError: invalid literal for int() with base 10: 'abc'

无法确定确切的类型

# 可以直接给出提示
try:
    int('abc')
    sum = 1 + '1'
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except:
    print('出错啦！')

出错啦！

# 也可以根据可能的类型来给出提示
try:
    sum = 1 + '1'
    ff = open('我为什么是一个文件.txt')
    print(ff.read())
    ff.close()
except (OSError,TypeError):
    print('出错啦！！')

13. 异常检测——try-finally

# 语法
'''
try:
    检测范围
except Exception [as reason]:
    出现异常（EXception）后的处理代码
finally:
    无论如何都会被执行的代码
'''

try:
    ff = open('我是一个文件.txt','w')
    print(ff.write('我存在了！'))
    sum = 1 + '1'
    ff.close()
except (OSError,TypeError):
    print('出错啦！！')

# 由于在创建这个文件之后出现了异常，而文件还未关闭，即文件未保存，则文件未写入任何东西
# 会打印出准备写入字符的个数

5
出错啦！！

try:
    ff = open('我是一个文件.txt','w')
    print(ff.write('我存在了！'))
    sum = 1 + '1'
except (OSError,TypeError):
    print('出错啦！！')
finally:
    # 这里写入关闭文件的操作，无论程序怎么报错，都会执行这里
    ff.close()

5
出错啦！！

总结：程序出现异常时，会先执行except里的语句，然后执行finally里的语句

14. 引出异常——raise

# 直接
raise

---------------------------------------------------------------------------

RuntimeError                              Traceback (most recent call last)

<ipython-input-35-9c9a2cba73bf> in <module>()
----> 1 raise

RuntimeError: No active exception to reraise

raise ZeroDivisionError

---------------------------------------------------------------------------

ZeroDivisionError                         Traceback (most recent call last)

<ipython-input-36-c62fc6984c64> in <module>()
----> 1 raise ZeroDivisionError

ZeroDivisionError: 

raise ZeroDivisionError('除数为0的异常')

---------------------------------------------------------------------------

ZeroDivisionError                         Traceback (most recent call last)

<ipython-input-38-bec5ebf736a5> in <module>()
----> 1 raise ZeroDivisionError('除数为0的异常')

ZeroDivisionError: 除数为0的异常