异常处理

当你的程序中出现异常情况时就需要异常处理。比如当你打开一个不存在的文件时。当你的程序中有一些无效的语句时，Python会提示你有错误存在。

>>> Print 'Hello World'
    File "", line 1
      Print 'Hello World'
                        ^
SyntaxError: invalid syntax

>>> print 'Hello World'
Hello World

1、try...except语句

#!/usr/bin/python
#-*- encoding:UTF-8 -*-
# Filename: try_except.py

import sys

try:
	s = raw_input('Enter something --> ')
except EOFError:#处理EOFError类型的异常
	print '\nWhy did you do an EOF on me?'
	sys.exit() # 退出程序
except:#处理其它的异常
	print '\nSome error/exception occurred.'
	
print 'Done'
raw_input('(按任意键结束)')

$ python try_except.py
Enter something -->
Why did you do an EOF on me?
(按任意键结束)

$ python try_except.py
Enter something --> Python is exceptional!
Done
(按任意键结束)

2、引发异常

#!/usr/bin/python
#-*- encoding:UTF-8 -*-
#文件名: raising.py

class ShortInputException(Exception):
	'''你定义的异常类。'''
	def __init__(self, length, atleast):
		Exception.__init__(self)
		self.length = length
		self.atleast = atleast

try:
	s = raw_input('请输入 --> ')
	if len(s) < 3:
		raise ShortInputException(len(s), 3)
	# raise引发一个你定义的异常
except EOFError:
	print '\n你输入了一个结束标记EOF'
except ShortInputException, x:#x这个变量被绑定到了错误的实例
	print 'ShortInputException: 输入的长度是 %d, \
		长度至少应是 %d' % (x.length, x.atleast)
else:
	print '没有异常发生.'
raw_input('(按任意键结束)')

$ python raising.py
请输入 -->
你输入了一个结束标记EOF
(按任意键结束)

$ python raising.py
请输入 --> --> ab
ShortInputException: 输入的长度是 2, 长度至少应是 3
(按任意键结束)

$ python raising.py
请输入 --> abc
没有异常发生.
(按任意键结束)

3、try...finally语句

#!/usr/bin/python
#-*- encoding:UTF-8 -*-
# Filename: finally.py

import time

try:
	f = file('poem.txt')
	while True: # 读文件的一般方法
		line = f.readline()
		if len(line) == 0:
			break
		time.sleep(2)#每隔两秒输出一行
		print line,
finally:
	f.close()
	print 'Cleaning up...closed the file'
raw_input('(按任意键结束)')

$ 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
(按任意键结束)