九、python中的异常处理

1 异常

　　当读取一个文件，这个文件并不存在时候，会引发异常，类似的，还有很多异常的情况。

2 简单的异常测试代码

#此处在shell中运行。
>>> s = input('Enter something --> ')
Enter something --> 
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    s = input('Enter something --> ')
EOFError: EOF when reading a line
>>> 

3 通过try  except处理异常

#输入15
Enter something --> 15
You entered 15
#输入ctrl+D
Enter something --> ^D
Why did you do an EOF on me?
按住ctrl+C
Enter something -->
You cancelled the operation.

4 自定义异常类抛出异常

# encoding=UTF-8
class ShortInputException(Exception):
    '''一个由用户定义的异常类'''
    def __init__(self, length, atleast):
        Exception.__init__(self)
        self.length = length
        self.atleast = atleast
try:
    text = input('Enter something --> ')
    if len(text) < 3:
        raise ShortInputException(len(text), 3)
# 其他工作能在此处继续正常运行
except EOFError:
    print('Why did you do an EOF on me?')
except ShortInputException as ex:
    print(('ShortInputException: The input was ' +
    '{0} long, expected at least {1}')
    .format(ex.length, ex.atleast))
else:
    print('No exception was raised.')
#结果
1 输入ha
Enter something --> ha
ShortInputException: The input was 2 long, expected at least 3
2 输入haha
Enter something --> haha
No exception was raised.
3 输入ctrl+c
EOFError: EOF when reading a line

5 with语句的用法

　　在 try 块中获取资源，然后在 finally 块中释放资源是一种常见的模式。因此，还有一个with 语句使得这一过程可以以一种干净的姿态得以完成。 代码如下：

with open("poem.txt") as f:
    for line in f:
        print(line, end='')
#结果   注意，源码文件和poem.txt文件在同一目录下hello world
haha

参考：《byte-of-python-chinese-edition》