Python异常处理

我们可以使用try..except语句来处理异常。我们把通常的语句放在try-块中，而把我们的错误处理语句放在except-块中。

1.处理异常

#!/usr/bin/python
# Filename: try_except.py
import sys
try:
        s = raw_input('Enter something --> ')
except EOFError:
        print '
Why did you do an EOF on me?'
        sys.exit() # exit the program
except:
        print '
Some error/exception occurred.'
        # here, we are not exiting the program
print 'Done'

运行结果

# ./try_except.py    enter
Enter something -->
Done
# ./try_except.py   ctrl+d
Enter something -->
Why did you do an EOF on me?

2.如何引发异常

#!/usr/bin/python
# Filename: raising.py
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) < 3:
                raise ShortInputException(len(s), 3)
# Other work can continue as usual here
except EOFError:
        print '
Why 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.'

运行结果

[root@host python]# ./raising.py
Enter something --> ff
ShortInputException: The input was of length 2, was expecting at least 3
[root@host python]# ./raising.py
Enter something --> 222
No exception was raised.

这里，我们创建了我们自己的异常类型，其实我们可以使用任何预定义的异常/错误。这个新的异常类型是ShortInputException类。它有两个域——length是给定输入的长度，atleast则是程序期望的最小长度。

3.使用finally

#!/usr/bin/python
# Filename: finally.py
import time
try:
        f = file('poem.txt')
        while True: # our usual file-reading idiom
                line = f.readline()
                if len(line) == 0:
                        break
                time.sleep(2)
                print line,
finally:
        f.close()
        print 'Cleaning up...closed the file'

运行结果

# ./finally.py
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!
Cleaning up...closed the file
# ./finally.py
Programming is fun
When the work is done
^CCleaning up...closed the file      --ctrl+c to break
Traceback (most recent call last):
  File "./finally.py", line 10, in <module>
    time.sleep(2)
KeyboardInterrupt

在程序运行的时候，按Ctrl-c中断/取消程序。我们可以观察到KeyboardInterrupt异常被触发，程序退出。但是在程序退出之前，finally从句仍然被执行，把文件关闭