上下文管理器

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

    def __enter__(self):
        print("触发enter")

    def __exit__(self, exc_type, exc_val, exc_tb):
        print(exc_tb,'
',exc_val,'
',exc_type)  #接收异常的三个参数
        print("退出exit")

with Open('a.txt') as f:
    print("===========")
    print("===========")
    # print(dfasdklf)  
    print("===========")
    '''
    输出:
        触发enter
        ===========
        ===========
        ===========
        None 
         None 
         None
        退出exit
    '''

下面，让程序产生异常：

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

    def __enter__(self):
        print("触发enter")

    def __exit__(self, exc_type, exc_val, exc_tb):
        print(exc_tb,'
',exc_val,'
',exc_type)  #接收异常的三个参数
        print("退出exit")

with Open('a.txt') as f:
    print("===========")
    print("===========")
    print(dfasdklf)
    print("===========")
    '''
    输出:
        触发enter
        ===========
        ===========   #总共两个，说明出现异常立即退出
        <traceback object at 0x0000014E7651D0C8> 
         name 'dfasdklf' is not defined 
         <class 'NameError'>
        退出exit
    '''

 自定义用于打开文件的上下文管理器：

class MyOpenClass:
    def __init__(self,name,mode):
        self.name= name
        self.mode= mode
    
    def __enter__(self):
        print("进入上下文管理器")
        if self.mode == 'r':
            self.f = open(self.name,self.mode)
        else:
            self.f= open(self.name,self.mode)
        return self #重点，要将self， 它返回的就是as 后面的那个
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()
        print("退出上下文管理器")
    def read(self):
        return self.f.read()
    def write(self,s):
        self.f.write(s)
if __name__ =="__main__":
    with MyOpenClass("d:/test.txt",'r') as f1,MyOpenClass("d:/test02.txt",'w') as f2:
        txt = f1.read() #从f1中读
        print(txt)
        f2.write(txt)  # 写到f2中去