要打開文件，我習慣照書本教的
fobj = open(filename, 'r')
怕文件不存在，可能還得再加個捕捉錯誤
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try:
fobj = open(filename, 'r')
except IOError:
pass
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但有了with as 就方便多了，可以
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with open(filename, 'r') as fobj:
do something
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為了讓class 也能使用，需添加二個函數__enter__和 __exit__
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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
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使用模組contextlib，便不用構造__enter__和 __exit__
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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
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官方範例
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from contextlib import contextmanager
@contextmanager
def tag(name):
print "<%s>" % name
yield
print "</%s>" % name
>>> with tag("h1"):
... print "foo"
...
<h1>
foo
</h1>
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還有一個closing可在抓網頁資料時使用
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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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