python学习笔记9——第八章 异常

qq78707

　　第八章  异常
　　1.
　　（1）捕捉异常，try & except

>>> try:
x = input("x: ")
y = input("y: ")
print x/y
except ZeroDivisionError:
print "The second number can't be zero!"

x: 1
y: 0
The second number can't be zero!
　　（2）捕捉异常并重新引发

>>> class MuffledCalculator:
muffled = False;  #屏蔽开关的标志
def calc(self, expr):
try:
return eval(expr)
except ZeroDivisionError:
if self.muffled:  #若屏蔽开着，则捕捉异常并进行处理
print "Division by zero is illegal"
else:  #若屏蔽关着，则捕捉异常并重新引发
raise  #except捕捉到了异常，raise重新引发异常

>>> calculator = MuffledCalculator()
>>> calculator.calc('10/2')
5
>>> calculator.calc('10/0') #屏蔽机制未打开，因为muffled = False
Traceback (most recent call last):
File "<pyshell#29>", line 1, in <module>
calculator.calc('10/0') #屏蔽机制未打开，因为muffled = False
File "<pyshell#26>", line 5, in calc
return eval(expr)
File "<string>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
>>> calculator.muffled = True  #打开屏蔽机制
>>> calculator.calc('10/0')
Division by zero is illegal
　　（3）多个except，捕捉多个不同类型的异常

>>> try:
x = input("x: ")
y = input("y: ")
print x/y
except ZeroDivisionError:  
print "Y can't be zero!"
except TypeError:
print "That wasn't a numbber, was it?"

x: 1
y: 'test'
That wasn't a numbber, was it?
　　（4）一个except捕捉多个类型异常，效果同（3）

>>> try:
x = input("x: ")
y = input("y: ")
print x/y
except(ZeroDivisionError, TypeError, NameError):  #NameError找不到名字（变量）时引发
print 'Your numbers were bogus(wrong)...'
　　（5）捕捉异常对象，想让程序继续运行，又想记录错误时应用

>>> try:
x = input("x: ")
y = input("y: ")
print x/y
except(ZeroDivisionError, TypeError), e:  #多个异常时也只需提供一个参数e，是一个元组
print "Invalid input:", e

x: 1
y: 0
Invalid input: integer division or modulo by zero
x: 1
y: 'test'
Invalid input: unsupported operand type(s) for /: 'int' and 'str'

　　（6）捕捉所有异常，except后不加东西

>>> try:
x = input("x: ")
y = input("y: ")
print x/y
except:
print "Something wrong happend..."

x:
Something wrong happend...
>>>
　　（7） try & except & else

>>> while True:
try:
x = input("x: ")
y = input("y: ")
print 'x/y is', x/y
except:
print "Invalid input. Please try again."
else:  #只有在异常没发生，也就是输入正确的时候，循环才退出
break

x: 1
y: 0
x/y is Invalid input. Please try again.
x: 2
y: 'test'
x/y is Invalid input. Please try again.
x:
Invalid input. Please try again.
x: 1
y: 2
 　　(8) try & finally， try & except & finally, try & except & else & finally
　　--不管是否发生异常，finally语句块一定会执行

>>> try:
1/0
except NameError:
print "Unknown variable"
else:
print "That went well!"
finally:
print "Cleaning up."
　　实例

>>> def describePerson(person):
print "Description of", person['name']
print 'Age:', person['age']
try:
print 'Occupation: ' + person['occupation']  #不能用逗号连接，若用逗号，则在异常发生前就打印了pccupation
except KeyError:  #键不存在
pass
>>> person = {'name': 'Sun', 'age': 32}
>>> describePerson(person)  #捕获异常并处理
Description of Sun
Age: 32
#如果最后一句为print 'Occupation:', person['occupation']
#则出现
>>> describePerson(person)
Description of Sun
Age: 32
Occupation:
#所以要用加号连接，不用逗号，防止在异常发生前就被打印。