8.1 什么是异常
每个异常都是一些类的实例
8.2 按自己的方式出错
8.2 raise语句
>>> raise Exception Traceback (most recent call last): File "", line 1, in Exception
>>> raise Exception('hyperdrive overload')
Traceback (most recent call last):
File "", line 1, in
Exception: hyperdrive overload
查看内建异常
>>> import exceptions >>> dir(exceptions) ['ArithmeticError', 'AssertionError',...]
8.2.2 自定义异常类
class SomeCustomException(Exception):pass
8.3 捕捉异常
try:
x=input('Enter the first number:')
y=input('Enter the second number:')
print x/y
except ZeroDivisionError:
print "The second number can't be zero!"
...
Enter the first number:10
Enter the second number:0
The second number can't be zero!
看，没参数
>>> class MuffedCalculator:
... muffled=False
... def calc(self,expr):
... try:
... return eval(expr)
... except ZeroDivisionError:
... if self.muffled:
... print 'Division by zero is illegal'
... else:
... raise
...
>>>
>>> calculator=MuffedCalculator()
>>> calculator.calc('10/2')
5
>>> calculator.calc('10/0')
Traceback (most recent call last):
File "", line 1, in
File "", line 5, in calc
File "", line 1, in
ZeroDivisionError: integer division or modulo by zero
>>> calculator.muffled=True
>>> calculator.calc('10/0')
Division by zero is illegal
8.4 不止一个except子句
>>> try:
... x=input('Enter the first number:')
... y=input('Enter the second number:')
... print x/y
... except ZeroDivisionError:
... print "The second number can't be zero!"
... except TypeError:
... print "That wasn't a number,was it?"
...
Enter the first number:10
Enter the second number:"Hello world!"
That wasn't a number,was it?
8.5 用一个会捕捉两个异常
>>> try:
... x=input('Enter the first number:')
... y=input('Enter the second number:')
... print x/y
... except (ZeroDivisionError,TypeError,NameError):
... print "Your numbers were bogus....."
...
Enter the first number:10
Enter the second number:0
Your numbers were bogus.....
8.6 捕捉对象
#访问异常对象本身
>>> try:
... x=input('Enter the first number:')
... y=input('Enter the second number:')
... print x/y
... except (ZeroDivisionError,TypeError,NameError),e:
... print "Your numbers were bogus....."
... print e
...
Enter the first number:10
Enter the second number:0
Your numbers were bogus.....
integer division or modulo by zero
8.7 真正的全捕捉
缺点：会隐藏所有程序员未想到并且未做好准备的错误
>>> try:
... x=input('Enter the first number:')
... y=input('Enter the second number:')
... print x/y
... except:
... print "Someting wrong happened...."
...
Enter the first number:
Someting wrong happened....
推荐用下面的方式：
>>> try:
... x=input('Enter the first number:')
... y=input('Enter the second number:')
... print x/y
... except Exception,e:
... print "Someting wrong happened...."
... print e
...
Enter the first number:10
Enter the second number:0
Someting wrong happened....
integer division or modulo by zero
8.8 万事大吉
else:没有异常的时候会执行
>>> try: ... print 'A simple task' ... except: ... print 'What?something went wrong?' ... else: ... print 'Ah....It went as planned..' ... A simple task Ah....It went as planned..
while True:
try:
x=input('Enter the first number:')
y=input('Enter the second number:')
value=x/y
print 'x/y is',value
except:
print 'Invalid input,Please try again.'
else:
break
while True:
try:
x=input('Enter the first number:')
y=input('Enter the second number:')
value=x/y
print 'x/y is',value
except Exception,e:
print 'Invalid input',e
print 'Please try again'
else:
break
8.9 最后....
finally：如果需要确保某些代码不管是否有异常都执行
>>> x=None >>> try: ... x=1/0 ... finally: ... print 'Cleaning up....' ... del x ... Cleaning up.... Traceback (most recent call last): File "", line 2, in ZeroDivisionError: integer division or modulo by zero
>>> try: ... 1/0 ... except NameError: ... print 'Unknown variable' ... else: ... print 'That went well!' ... finally: ... print 'Cleaning up.' ... Cleaning up. Traceback (most recent call last): File "", line 2, in ZeroDivisionError: integer division or modulo by zero
8.10 异常和函数
>>> def faulty():
... raise Exception('Someting is wrong')
...
>>> def ignore_exception():
... faulty()
...
>>> def handle_exception():
... try:
... faulty()
... except:
... print 'Exception handled'
...
>>> ignore_exception() Traceback (most recent call last): File "", line 1, in File " ", line 2, in ignore_exception File " ", line 2, in faulty Exception: Someting is wrong >>> handle_exception() Exception handled
8.11 异常之禅
使用try/except 语句比if/else语句更自然一些
>>> def describePersion(person): ... print 'Description of',person['name'] ... print 'age:',person['age'] ... if 'occupation' in person: ... print 'Occupation:',person['occupation'] ...
>>> person={'name':'jack','age':10,'occupation':'camper'}
>>> describePersion(person)
Description of jack
age: 10
Occupation: camper
>>> person1={'name':'jack','age':10}
>>> describePersion(person1)
Description of jack
age: 10
>>> def describePersion(person): ... print 'Description of',person['name'] ... print 'age:',person['age'] ... try: ... print 'Occupation:'+person['occupation'] ... except KeyError:pass ... >>> >>> describePersion(person) Description of jack age: 10 Occupation:camper >>> describePersion(person1) Description of jack age: 10
try: obj.write except AttributeError: print "The object is not writeable" else： print 'The Object is writeable'