__enter__和__exit__上下文管理协议

# __enter__和__exit__上下文管理协议

f = open('test.txt', 'r', encoding='utf-8')
f.read()
f.close()
# 等同于下面的with...as...写法，实际上就是实现了上下文管理协议，无需再关闭f对象
with open('test.txt', 'r', encoding='utf-8') as f:
    f.read()


class Open:
    def __init__(self, name):
        self.name = name

    def __enter__(self):
        print('执行enter')
        return self

    # with中的代码块出现异常时或执行完with后就会调用__exit__
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('执行exit')
        print(exc_type)  # 异常类名
        print(exc_val)  # 异常的值
        print(exc_tb)  # 异常的追踪信息
        return True  # return True时，当with中的代码块出现异常时，程序不会报错


with Open('test.txt') as f:
    pass