上下文管理协议 | 易学教程

__enter__    (输入)  和__exit__    （出口）

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 Open('a.txt') as f:
    print(f)
    print(f.name)
    print('----------->')
    print('----------->')
print('000000000000')

执行enter #__enter__ 运行
<__main__.Open object at 0x02F3E118>
a.txt
----------->
----------->
执行exit #__exit__ 运行
000000000000 #with运行完毕运行其他语句

__exit__(self, exc_type, exc_val, exc_tb)

exc_type, exc_val, exc_tb分别代表什么？

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(exc_type)
        print(exc_val)
        print(exc_tb)


with Open('a.txt') as f:
    print(f)
    print(f.sdadsadsa)
    print(f.name)
    print('----------->')
    print('----------->')
    print('----------->')
    print('----------->')
    print('----------->')
print('000000000000')

执行enter #执行__enter__
Traceback (most recent call last):
<__main__.Open object at 0x0365E118> #执行print（f）
File "D:/pycharm/python_s3/2020-1-2/上下文管理协议.py", line 18, in <module>
执行exit　　　　　　　　　　　　　　　　　　　　　　　　　#出错执行__exit__
print(f.sdadsadsa)　　　　　　　　　　　　　　　　　　
<class 'AttributeError'>　　　　　　　　　　　　　　　#执行print(exc_type)
AttributeError: 'Open' object has no attribute 'sdadsadsa'
'Open' object has no attribute 'sdadsadsa' #执行print(exc_val)
<traceback object at 0x036DD088>　　　　　　　　　　 #执行print(exc_tb)

__exit__   添加 return Truewith碰到出错直接结束    运行with之后的语句

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(exc_type)
        print(exc_val)
        print(exc_tb)
        return True


with Open('a.txt') as f:
    print(f)
    print(f.sdadsadsa)
    print(f.name)
    print('----------->')
    print('----------->')
    print('----------->')
    print('----------->')
    print('----------->')
print('000000000000')

执行enter
<__main__.Open object at 0x0300E118>
执行exit
<class 'AttributeError'>
'Open' object has no attribute 'sdadsadsa'
<traceback object at 0x0308C088>
000000000000

来源：https://www.cnblogs.com/hb15988111121/p/12133337.html

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