Python全栈day28（上下文管理）

　　我们知道在操作文件对象的时候可以这么写

1 2	`with` `open('a.txt',''r) as f:` `代码`

　　上述叫做上下文管理协议，即with语句，为了让一个对象兼容with语句，必须在这个对象的类中声明__enter__和__exit__方法

　　上下文管理协议.py

class Open:
    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')
 
#with触发的是__enter__
#返回的是self
with Open('a.txt') as f:
    print('===>')
    print('===>')
    print('===>')
    print(f)
    print(f.name)
#执行完毕with会触发__exit__的运行

　　正常是在with Open的时候执行__enter__然后执行with下面代码，如果执行代码没有出错exc_type, exc_val, exc_tb的值均为None

　　假如在with下面执行出现错误则直接跳到__exit__并且不执行往下的代码

　　上下文管理协议2.py

class Open:
    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('这是exc_type',exc_type)
        print('这是exc_cal',exc_val)
        print('这是exc_tb',exc_tb)
 
#with触发的是__enter__
#返回的是self
with Open('a.txt') as f:
    #f是self
    print(f)
    print(asdaddgdfg)
    #打印出对应的属性
    print(f.name)
#执行完毕with会触发__exit__的运行
print('0000000')

　　exc_type, exc_val, exc_tb 三个参数分别对应异常类，异常值，异常追踪信息

　　如果在__exit__返回一个True则不会报错而把对应的错误输出，不会执行with接着的语句print(f.name)，但是会继续执行with下面的语句print('0000000')

lass Open:
    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('这是exc_type',exc_type)
        print('这是exc_cal',exc_val)
        print('这是exc_tb',exc_tb)
        return True
#with触发的是__enter__
#返回的是self
with Open('a.txt') as f:
    #f是self
    print(f)
    print(asdaddgdfg)
    #打印出对应的属性
    print(f.name)
#执行完毕with会触发__exit__的运行
print('0000000')