python中关于with及contextlib的用法

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with file(‘test.py‘,‘r‘) as f :

    print f.readline()

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#!encoding:utf-8

class echo :

    def output(self) :

        print ‘hello world‘

    def __enter__(self):

        print ‘enter‘

        return self #返回自身实例，当然也可以返回任何希望返回的东西

    def __exit__(self, exception_type, exception_value, exception_traceback):

        #若发生异常，会在这里捕捉到，可以进行异常处理

        print ‘exit‘

        #如果改__exit__可以处理改异常则通过返回True告知该异常不必传播，否则返回False

        if exception_type == ValueError :

            return True

        else:

            return False

with echo() as e:

    e.output()

    print ‘do something inside‘

print ‘-----------‘

with echo() as e:

    raise ValueError(‘value error‘)

print ‘-----------‘

with echo() as e:

    raise Exception(‘can not detect‘)

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from contextlib import contextmanager

@contextmanager

def make_context() :

    print ‘enter‘

    try :

        yield {}

    except RuntimeError, err :

        print ‘error‘ , err

    finally :

        print ‘exit‘

with make_context() as value :

    print value

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from contextlib import contextmanager

from contextlib import nested

from contextlib import closing

@contextmanager

def make_context(name) :

    print ‘enter‘, name

    yield name

    print ‘exit‘, name

with nested(make_context(‘A‘), make_context(‘B‘)) as (a, b) :

    print a

    print b

with make_context(‘A‘) as a, make_context(‘B‘) as b :

    print a

    print b

class Door(object) :

    def open(self) :

        print ‘Door is opened‘

    def close(self) :

        print ‘Door is closed‘

with closing(Door()) as door :

    door.open()