【python自学】（九）-----异常

处理异常

使用try...except语句来处理异常。

import sys

try:
    s = raw_input('Enter something --> ')
except EOFError:
    print '\nWhy did you do an EOF on me?'
    sys.exit() # exit the program
except:
    print '\nSome error/exception occurred.'
    # here, we are not exiting the program

print 'Done'

$ python try_except.py
Enter something -->
Why did you do an EOF on me?

$ python try_except.py
Enter something --> Python is exceptional!
Done

如果某个错误或异常没有被处理，默认的python处理器被调用，终止程序的运行，并且打印一个消息。

引发异常

使用raise语句引发异常，并且要指明异常的名称和伴随异常触发的异常对象，恶意引发的错误或者异常应该分别是一个error或exception类的直接或者间接导出类。

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) < 3:
        raise ShortInputException(len(s), 3)
    # Other work can continue as usual here
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.'

$ python raising.py
Enter something -->
Why did you do an EOF on me?

$ python raising.py
Enter something --> ab
ShortInputException: The input was of length 2, was expecting at least 3

$ python raising.py
Enter something --> abc
No exception was raised.

try..finally

当读一个文件时，希望在无论异常发生与否的情况下都关闭文件，使用finally块完成。在一个try块下，可以同时使用except从句和finally块。

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'

$ python finally.py
Programming is fun
When the work is done
Cleaning up...closed the file
Traceback (most recent call last):
  File "finally.py", line 12, in ?
    time.sleep(2)
KeyboardInterrupt

【python自学】（九）-----异常

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