__enter__和__exit__的用法

Posted 2020-09-14 study_python

#__enter__与__exit__是成对出现的,一般是在进行with obj时才会触发它们

class Open:
    def __init__(self,filepath,mode=‘r‘,encode=‘utf-8‘):
        self.f=open(filepath,mode=mode,encoding=encode)
        # self.filepath =filepath
        # self.mode =mode
        # self.encoding =encode
    def __getattr__(self, item):
         return getattr(self.f,item)
    def __enter__(self):
        return self #如果是self.f,则在f.write(),就不会执行__getattr__方法

    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

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

#f =Open(‘a.txt‘,‘w‘)
with Open(‘a‘,‘w‘) as f:
    print(f)
    f.write(‘aaaaa\n‘)  #f.write(item),因为对象f没有write()方法,就执行__getattr__方法
    #即getattr(f.x,write)(item)
    f.write(‘bbbb‘)