python with用法

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python中with可以明显改进代码友好度，比如：

[python] view plaincopyprint?

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

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：

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>>> 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

>>> 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 enterin within 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：

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>>> 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

>>> 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

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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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