Exception

exceptions_handle.py

# 异常的处理
# try...except
# 通常的语句在try块里  错误处理器放在except块里


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))
    
# 如果 except 后面不指定错误名称 那么就是处理所有错误
# else 将在没有发生异常的时候执行

exceptions_raise.py

# 引发raise异常
# 要提供错误名或异常名以及要抛出Thrown的对象

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, excepted at least {1}'.format(ex.length,ex.atleast))
else:
    print('No exception was raised.')

exceptions_finally.py

# try ... finally
# 主要是用在文件里面

import sys
import time

f = None
try:
    f = open('poem.txt')
    
    while True:
        line = f.readline()
        if len(line) == 0:
            break
        print(line,end='')
        # 以便print的内容可以被立即打印到屏幕上
        sys.stdout.flush()
        print('Press ctrl+c now')
        
        # 每打印一行后插入两秒休眠，让程序运行得比较缓慢
        time.sleep(2)
except IOError:
    print('Could not find file poem.txt')
except KeyboardInterrupt:
    print('!! You cancelled the reading from the file.')
finally:
    if f:   # 这句话记得要
        f.close()
    print('(Clearning up:Closed the file)')

exceptions_using_with.py

# 在try块中获取资源（也就是open） 然后在finally块中释放资源（也就是close()）
# with语句可以很简单做到这一模式

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

# 文件的关闭交由with ... open来完成
# with 会获取open返回的对象f
# 然后在代码块开始之前调用 f.__enter__函数 
# 在代码块结束之后调用 f.__exit__函数