上下文协议管理

with obj as f:
'代码块'

1.with obj ----》触发obj.__enter__(),拿到返回值

2.as f----->f=返回值、

3.with obj as f 等同于 f=obj.__enter__()

4.执行代码块
一：没有异常的情况下，整个代码块运行完毕后去触发__exit__,它的三个参数都为None
二：有异常的情况下，从异常出现的位置直接触发__exit__
a：如果__exit__的返回值为True，代表吞掉了异常
b：如果__exit__的返回值不为True，代表吐出了异常
c：__exit__的的运行完毕就代表了整个with语句的执行完毕

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

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

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('执行exit')
        print(exc_type)
        print(exc_val)
        print(exc_tb)
        return True
with Foo('a.txt') as f:
    print(f)
    print(asdfsaasdfasdfasdfasdfasfasdfasdfasdfasdfasfdasfd)  #触发__exit__
    print(f.name)
    print('-----------------')
    print('-----------------')
    print('-----------------')
    print('-----------------')
    print('-----------------')
    print('-----------------')
    print('-----------------')
print('000000000000000000000000000000000000000000000')