15.5 with语句

#with语句确保，不论是否运行错误都确保文件关闭，

with open('a.txt','r') as file:
    print(file.read())

#

MyContentMgr实现了特殊方法__enter__(),__exit__()称为该类对象遵守了上下文管理器协议
该类对象的实例对象，称为上下文管理器

MyContentMgr()

class MyContentMgr(object):
    def __enter__(self):
        print('enter方法被调用执行了')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):            
        print('exit方法被调用执行了')

    def show(self):
        print('show方法被调用执行了',1/0)

with  MyContentMgr() as file:                  #相当于file=MyContentMgr()
    file.show()

#以后复制文件代码

with open('logo.png','rb') as src_file:                   #打开复制完关闭
    with open('copy2logo.png','wb') as target_file:       #打开写完，关闭
        target_file.write(src_file.read())