使用try...except语句来处理异常。我们把通常的语句放在try-块中,而把错误处理语句放在except-块中。
import sys try:
s = raw_input('Enter something-->')
except EOFError:
print '\nWhy did you do an EOF on me?'
sys.exit()
except:
print'\nSome error/exception occurred.' print 'Done'
结果:
==================== RESTART: D:/python_test/pickling.py ====================
Enter something--> Why did you do an EOF on me?
>>>
==================== RESTART: D:/python_test/pickling.py ====================
Enter something-->
Done
把所有可能引发错误的语句放在try块中,然后在except从句/块中处理所有的错误和异常。
except从句可以专门处理单一的错误或异常,或者一组包括在圆括号内的错误/异常。
如果没有给出错误或者异常的名称,它会处理所有的错误和异常。
对于每个try从句,至少都有一个相关联的except从句。
如果某个错误或异常没有被处理,默认的python处理器就会被调用。它会终止程序的运行,并且打印一个消息。
还可以让try..catch块关联上一个else从句。当没有异常发生的时候,else从句将被执行。
引发异常:
可以使用raise语句引发异常,你需要指明错误/异常的名称和伴随异常触发的异常对象。你可以引发的错误和异常应该分别是一个Error或Exception类的直接或间接导出类。
class ShortInputException(Exception):
'A user-defined exception class.'
def __init__(self,length,atleast):
Exception.__init__(self)
self.length=length
self.atleast=atleast try:
s=raw_input('Enter something-->')
if len(s)<:
raise ShortInputException(len(s),)
except EOFError:
print '\nWhy did you do an EOF on me?'
except ShortInputException,x:
print 'ShortInputException: The input was of length %d,\
was expecting at least %d'%(x.length,x.atleast)
else:
print 'No exception was raised.'
结果:
==================== RESTART: D:/python_test/pickling.py ====================
Enter something--> Why did you do an EOF on me?
>>>
==================== RESTART: D:/python_test/pickling.py ====================
Enter something-->ab
ShortInputException: The input was of length , was expecting at least
>>>
==================== RESTART: D:/python_test/pickling.py ====================
Enter something-->abc
No exception was raised.
try...finally
假如你在读一个文件的时候,希望在无论异常发生与否的情况下都关闭文件。我们可以使用finally块来完成。
注意,在一个try块下,你可以同时使用except从句和finally块,同时使用它们的时候,需要把一个嵌入另一个中
import time
try:
f=file('poem.txt')
while True:
line = f.readline()
if len(line)==:
break
time.sleep()
print line,
finally:
f.close()
print 'Cleaning up...closed the file'
结果:
programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
Cleaning up...closed the file
说明:在每打印一行之前用time.sleep方法暂停2秒。程序在运行过程中,按Ctrl-c中断/取消程序。
我们可以回看到这样:
programming is fun
When the work is done
Cleaning up...closed the file Traceback (most recent call last):
File "D:/python_test/pickling.py", line 64, in <module>
time.sleep(2)
KeyboardInterrupt
KeyboardInterrupt异常被触发,程序退出。但在程序退出之前,finally从句仍然被执行,把文件关闭