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