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