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python中with可以明显改进代码友好度,比如:

复制代码 代码如下:

with open('a.txt') as f: 
    print f.readlines() 

为了我们自己的类也可以使用with, 只要给这个类增加两个函数__enter__, __exit__即可:
复制代码 代码如下:

>>> class A: 
    def __enter__(self): 
        print 'in enter' 
    def __exit__(self, e_t, e_v, t_b): 
        print 'in exit' 
 
>>> with A() as a: 
    print 'in with' 
 
in enter 
in with 
in exit 

另外python库中还有一个模块contextlib,使你不用构造含有__enter__, __exit__的类就可以使用with:
复制代码 代码如下:

>>> from contextlib import contextmanager 
>>> from __future__ import with_statement 
>>> @contextmanager 
... def context(): 
...     print 'entering the zone' 
...     try: 
...         yield 
...     except Exception, e: 
...         print 'with an error %s'%e 
...         raise e 
...     else: 
...         print 'with no error' 
... 
>>> with context(): 
...     print '----in context call------' 
... 
entering the zone 
----in context call------ 
with no error 

使用的最多的就是这个contextmanager, 另外还有一个closing 用处不大
复制代码 代码如下:

from contextlib import closing 
import urllib 
 
with closing(urllib.urlopen('http://www.python.org')) as page: 
    for line in page: 
        print line 
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