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 "
     
      ", line 1, in 
      
        s = input('Enter something --> ') EOFError: EOF when reading a line >>>
      
     

3 通过try  except处理异常

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

4 自定义异常类抛出异常

# encoding=UTF-8class 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 = atleasttry:    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haEnter something --> haShortInputException: The input was 2 long, expected at least 32 输入hahaEnter something --> hahaNo exception was raised.3 输入ctrl+cEOFError: 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 worldhaha

参考：《byte-of-python-chinese-edition》