try:

    f = open('xxx', 'w')

    # write

    ...

finally:

    f.close()

with open('xxx', 'r') as f:

    # write

    ...

# encoding=utf-8

path = '/tmp/temp.txt'

with open(path, 'w') as f:

    f.writelines(['hello'])

print '在with之外, 变量f仍然存在且可被访问:', f

print '但是, 文件流已经被关闭了: ', f.closed

在with之外, 变量f仍然存在且可被访问: <closed file '/tmp/temp.txt', mode 'w' at 0x7ff7f4076660>

但是, 文件流已经被关闭了:  True

with slim.arg_scope(

    [slim.conv2d],

    activation_fn=tf.nn.relu,

    weights_initializer= weights_initializer,

    biases_initializer = biases_initializer):

    with slim.arg_scope(

        [slim.conv2d, slim.max_pool2d],

                padding='SAME',

                data_format = self.data_format):

        ...

# encoding=utf-8

# !/usr/local/bin/Python3

class reverse_print():

    def __enter__(self):

        # 这个方法在进入with block时执行.

        print('enter with block.')

        import sys

        self.origin_write = sys.stdout.write

        sys.stdout.write = self.new_print# Python2里这个对象不可修改, Python3可以

        return 'HELLO, I AM WITH'# 被 as 接收

    def new_print(self, s):

        self.origin_write(s[::-1])

    def __exit__(self, exception_type, exception_code, traceback):

        # 跳出with block时执行.

        # 处理异常(这儿就忽略吧), 恢复进入with时的修改

        import sys

        sys.stdout.write = self.origin_write

        print('exit with block')

print('before with:', 'hello')

with reverse_print() as cmo:

    print('hello')

    print(cmo)

print('after with:','hello')

print(cmo)

before with: hello

enter with block.

olleh

HTIW MA I ,OLLEH

exit with block

after with: hello

HELLO, I AM WITH