python基础----实现上下文管理协议__enter__和__exit__

我们知道在操作文件对象的时候可以这么写

with open('a.txt') as f:
　　'代码块'

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

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

     def __enter__(self):
         print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')
         # return self
     def __exit__(self, exc_type, exc_val, exc_tb):
         print('with中代码块执行完毕时执行我啊')

 with Open('a.txt') as f:
     print('=====>执行代码块')
     # print(f,f.name)

 上下文管理协议

上下文管理协议

__exit__()中的三个参数分别代表异常类型，异常值和追溯信息,with语句中代码块出现异常，则with后的代码都无法执行

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

     def __enter__(self):
         print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')

     def __exit__(self, exc_type, exc_val, exc_tb):
         print('with中代码块执行完毕时执行我啊')
         print(exc_type)
         print(exc_val)
         print(exc_tb)

 with Open('a.txt') as f:
     print('=====>执行代码块')
     raise AttributeError('***着火啦,救火啊***')
 print(''*100) #------------------------------->不会执行

如果__exit()返回值为True,那么异常会被清空，就好像啥都没发生一样，with后的语句正常执行

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

     def __enter__(self):
         print('出现with语句,对象的__enter__被触发,有返回值则赋值给as声明的变量')

     def __exit__(self, exc_type, exc_val, exc_tb):
         print('with中代码块执行完毕时执行我啊')
         print(exc_type)
         print(exc_val)
         print(exc_tb)
         return True

 with Open('a.txt') as f:
     print('=====>执行代码块')
     raise AttributeError('***着火啦,救火啊***')
 print(''*100) #------------------------------->会执行

用途或者说好处：

1.使用with语句的目的就是把代码块放入with中执行，with结束后，自动完成清理工作，无须手动干预

2.在需要管理一些资源比如文件，网络连接和锁的编程环境中，可以在__exit__中定制自动释放资源的机制，你无须再去关系这个问题，这将大有用处

-----------------------------------------------------------------------------------------------------------------

笔记:

# with open('a.txt','r') as f:
#     print('--=---->')
#     print(f.read())

# with open('a.txt', 'r'):
#     print('--=---->')

#
class Foo:
    def __enter__(self):
        print('=======================》enter')
        return 111111111111111

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('exit')
        print('exc_type',exc_type)
        print('exc_val',exc_val)
        print('exc_tb',exc_tb)
        return True

# with Foo(): #res=Foo().__enter__()
#     pass

with Foo() as obj: #res=Foo().__enter__() #obj=res
    print('with foo的自代码块',obj)
    raise NameError('名字没有定义')
    print('************************************')

print('')

上下文管理协议

 import time
 class Open:
     def __init__(self,filepath,mode='r',encode='utf-8'):
         self.f=open(filepath,mode=mode,encoding=encode)

     def write(self,line):
         self.f.write(line)

     def __getattr__(self, item):
         return getattr(self.f,item)

     def __del__(self):
         print('----->del')
         self.f.close()

     def __enter__(self):
         return self.f
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.f.close()

 with Open('egon.txt','w') as f:
     f.write('egontest\n')
     f.write('egontest\n')
     f.write('egontest\n')
     f.write('egontest\n')
     f.write('egontest\n')

 class Open:
     def __init__(self,filepath,mode,encode='utf-8'):
         self.f=open(filepath,mode=mode,encoding=encode)
         self.filepath=filepath
         self.mode=mode
         self.encoding=encode

     def write(self,line):
         print('write')
         self.f.write(line)

     def __getattr__(self, item):
         return getattr(self.f,item)

     def __enter__(self):
         return self

     def __exit__(self, exc_type, exc_val, exc_tb):
         self.f.close()
         return True

 with Open('aaaaa.txt','w') as write_file: #write_file=Open('aaaaa.txt','w')
     write_file.write('123123123123123\n')
     write_file.write('123123123123123\n')
     print(sssssssssssssss)
     write_file.write('123123123123123\n')

上下文管理协议01