Python入门（简明Python教程）——申

异常处理

try:
    text=input('Enter something-->')
except EOFError:
    print('Why did you do an EOF on me?')
except KeyboardInterrupt:
    print('You cancelled the operation.')
else:
    print('You entered {}'.format(text))

结果：输入“asd”

Enter something-->asd
You entered asd

结果：键入“Ctrl+d”

Enter something-->Why did you do an EOF on me?

抛出异常：

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)
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：

Enter something-->asd
No exception was raised.

Enter something-->a
ShortInputException: The input was 1 long,expected at least 3

无异常的即“else”

另一个例子（注意，ctrl+c要在命令行中才有用）

import sys
import time
f=None
try:
    f=open('poem.txt')
    while True:
        line=f.readline()
        if len(line)==0:
            break
        print(line,end=' ')
        sys.stdout.flush()
        print('Press ctrl+c now')
        time.sleep(2)
except IOError:
    print("can't find")
except KeyboardInterrupt:
    print("cancel reading")
finally:
    if f:
        f.close()
    print("(Cleaning up:Closed the file)")
input()

正如上面这样，一般选择在try中打开资源，而在finally里释放资源

两种读文本的方式：

with open('poem.txt')as f:
    for line in f:
        print(line,end=' ')


f=open('poem.txt')
while True:
    line=f.readline()
    if(len(line)==0):
        break
    print(line,end=' ')
f.close()

可以看到，with语法更为简单