python中with可以明顯改進代碼友好度，比如：
複製代碼 代碼如下:
with open('a.txt') as f:
print f.readlines()
為了我們自己的類也可以使用with， 只要給這個類增加兩個函數__enter__, __exit__即可：
複製代碼 代碼如下:
>>> 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庫中還有一個模組contextlib，使你不用構造含有__enter__, __exit__的類就可以使用with：
複製代碼 代碼如下:
>>> 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
使用的最多的就是這個contextmanager, 另外還有一個closing 用處不大
複製代碼 代碼如下:
from contextlib import closing
import urllib
with closing(urllib.urlopen('http://www.python.org')) as page:
for line in page:
print line
