python with原理

在python2.5+中可以用with来保证关闭打开的文件

with open(hello.txt) as f:
    do some file operations

为什么要引入with呢?

在之前如果要保证关闭文件需要这样: 

f = open(hello.txt)
try:
  do some file operations
finally:
  f.close()

为了简化这些操作,pyhton2.5中引入with,在进入with时python会自动调用对象的__enter__ 函数,

离开with时会调用 __exit__ 函数,如下所示。

class SomeThing(object):
    def __enter__(self):
        set things up
        return self
    def __exit__(self, type, value, traceback):
        tear things down

是不是很简单?

比如我们可以实现一个自动释放资源的connection类

class Connection(object):
    def __enter__(self):
    print enter
    return self

    def __exit__(self, type, value, traceback):
    print exit

    def say_hi(self):
    print hello, world

def open_connection():
    return Connection()

with open_connection() as conn:
    conn.say_hi()

程序输出为:

enter
hello, world
exit

 

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python with原理

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