python with用法

python中with可以明显改进代码友好度，比如：

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：

>>> 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：

>>> 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

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