第8章 异常
8.1 什么是异常
>>> 1/0
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
1/0
ZeroDivisionError: integer division or modulo by zero
8.2按自己的方式出错
8.2.1 raise语句
>>> raise Exception
Traceback (most recent call last):
File "<pyshell#1>", line 1, in <module>
raise Exception
Exception
>>> raise Exception('hyperdrive overload')
Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
raise Exception('hyperdrive overload')
Exception: hyperdrive overload
>>> import exceptions
>>> dir(exceptions)
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'DeprecationWarning', 'EOFError', 'EnvironmentError', 'Exception', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'NameError', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'ReferenceError', 'RuntimeError', 'RuntimeWarning', 'StandardError', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'WindowsError', 'ZeroDivisionError', '__doc__', '__name__']
>>> raise ArithmeticError
Traceback (most recent call last):
File "<pyshell#7>", line 1, in <module>
raise ArithmeticError
ArithmeticError
>>>
8.3 捕捉异常
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except ZeroDivisionError:
print 'The second number should not be zero! '
Input the x: 10
Input the y: 0
The second number should not be zero!
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except ZeroDivisionError:
print 'The second number should not be zero! '
Input the x: 8
Input the y: 9
0
屏蔽除零错误
>>> class MuffledCalculator:
muffled = False
def calc(self,expr):
try:
return eval(expr)
except ZeroDivisionError:
if self.muffled:
print 'Division by zero is illegal'
else:
raise
>>> calculator = MuffledCalculator()
>>> calculator.calc('10/2')
5
>>> calculator.calc('10/0')
Traceback (most recent call last):
File "<pyshell#40>", line 1, in <module>
calculator.calc('10/0')
File "<pyshell#37>", line 5, in calc
return eval(expr)
File "<string>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> calculator.muffled = True
>>> calculator.calc('10/0')
Division by zero is illegal
如果捕捉到异常，但是又想重新引发它，也就是说要传递异常，那么可以调用不带参数的raise们还能在捕捉到异常时显式的提供具体异常
如果这个行为被激活，那么计算器就会打印错误信息，而不是让异常传播，如果在于用户交互的过程中，那么这就很有用了。
但是如果在程序内部使用，引发异常会更好些，因此屏蔽机制就可以关掉了。
8.4 不止一个except子句
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except ZeroDivisionError:
print 'The second number should not be zero! '
except TypeError:
print 'That was not a number?!'
Input the x: 10
Input the y: 'Hi'
That was not a number?!
8.5 用一个块捕捉两个异常
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except (ZeroDivisionError, TypeError, NameError):
print 'Your numbers were bogus....'
Input the x: 8
Input the y: 0
Your numbers were bogus....
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except (ZeroDivisionError, TypeError, NameError):
print 'Your numbers were bogus....'
Input the x: 10
Input the y: 'Hi'
Your numbers were bogus....
8.6捕捉对象
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except (ZeroDivisionError, TypeError),e:
print e
Input the x: 10
Input the y: 0
integer division or modulo by zero
>>> e
ZeroDivisionError('integer division or modulo by zero',)
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except (ZeroDivisionError, TypeError),e:
print e
Input the x: 'Hello'
Input the y: 'world'
unsupported operand type(s) for /: 'str' and 'str'
>>> e
TypeError("unsupported operand type(s) for /: 'str' and 'str'",)
8.7真正的全捕捉
>>> try:
x = input('Input the x: ')
y = input('Input the y: ')
print x/y
except :
print 'Something was wrong...'
Input the x: 8
Input the y: 'haha'
Something was wrong...
8.8 完事大吉
>>> try:
print 'A simple task.'
except:
print 'What? Something was wrong?'
else:
print 'Ah...It was as planned'
A simple task.
Ah...It was as planned
一直输入直到合法输入
>>> while True:
try:
x = input('Input the x: ')
y = input('Input the y: ')
value = x/y
print 'x/y is',value
except:
print 'Invalid input.Please try again'
else:
break
Input the x: 8
Input the y: 0
Invalid input.Please try again
Input the x: 8
Input the y: '1'
Invalid input.Please try again
Input the x: 8
Input the y:
Invalid input.Please try again
Input the x: 9
Input the y: 2
x/y is 4
打印错误信息
>>> while True:
try:
x = input('Input the x: ')
y = input('Input the y: ')
value = x/y
print 'x/y is',value
except Exception , e:
print 'Invlid input',e
print 'Invalid input.Please try again'
else:
break
Input the x: 1
Input the y: 0
Invlid input integer division or modulo by zero
Invalid input.Please try again
Input the x: 4
Input the y: '2'
Invlid input unsupported operand type(s) for /: 'int' and 'str'
Invalid input.Please try again
Input the x: 3
Input the y:
Invlid input unexpected EOF while parsing (<string>, line 0)
Invalid input.Please try again
Input the x: 4
Input the y: 2
x/y is 2
8.9 最后
>>> x = None
>>> try:
x = 1/0
finally:
print 'Cleaning up...'
del x
Cleaning up...
Traceback (most recent call last):
File "<pyshell#6>", line 2, in <module>
x = 1/0
ZeroDivisionError: integer division or modulo by zero
>>>
>>> try:
1/0
except NameError:
print 'Unkown variable'
else:
print 'It is ok.'
finally:
print 'Cleaning up..'
Cleaning up..
Traceback (most recent call last):
File "<pyshell#17>", line 2, in <module>
1/0
ZeroDivisionError: integer division or modulo by zero
>>>
8.10 异常和函数
def faulty():
raise Exception('Something is wrong.')
def ignore_exception():
faulty()
def handle_excepton():
try:
faulty()
except:
print 'Exception handled'
print 'ignore_exception()'
ignore_exception()
ignore_exception()
Traceback (most recent call last):
File "F:/Python/8-1函数与异常.py", line 11, in <module>
ignore_exception()
File "F:/Python/8-1函数与异常.py", line 4, in ignore_exception
faulty()
File "F:/Python/8-1函数与异常.py", line 2, in faulty
raise Exception('Something is wrong.')
Exception: Something is wrong.
>>> handle_exception()
Exception handled
>>>
8.11
>>> def describlePerson(person):
print 'Description of ',person['name']
print 'Age: ',person['age']
try:
print 'Occupation: '+person['occupation']
except KeyError:
pass
>>> person = {'name':'ABC','age':21}
>>> person
{'age': 21, 'name': 'ABC'}
>>> describlePerson(person)
Description of ABC
Age: 21