魔法方法《六》__enter__和__exit__

https://www.cnblogs.com/DragonFire/p/6764066.html

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class Sample():

    def __enter__(self):
        # 获取资源
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        # 释放资源
        print('exit')

    def do_something(self):
        print('doing something')

with Sample() as sample:
    sample.do_something()