#!/usr/bin/python3 import weakrefimport sys  class Data(object): def __init__(self, value, owner):  self.value = value     # 聲明弱引用，owner為Node類本身  self.owner = weakref.ref(owner)   # 通過函數(shù)調(diào)用的方式訪問引用對象 def __str__(self):  return "%s's data, value is %s" % (self.owner(), self.value)   def __del__(self):  print('in_data.__del__')  class Node(object): def __init__(self, value):     # 把類本身，也當做參數(shù)傳入Data類中  self.data = Data(value, self)   # 自定義對象名，容易辨認 def __str__(self):  return 'Node'   def __del__(self):  print('in_node.__del__')   if __name__ == '__main__': node = Node(100) print(node.data)   # 打印node對象的引用計數(shù) print(sys.getrefcount(node) - 1)   # 當刪除node對象時候，Data實例對象在引用計算為0也相應釋放 del node   input('del done >>>>>')