1. class Singleton(type): 
  2.     def __init__(self, *args, **kwargs):
  3.         self.__instance = None
  4.         super().__init__(*args, **kwargs) # 继承type的所有属性
  6.     def __call__(self, *args, **kwargs):
  7.         if self.__instance is None:
  8.             self.__instance = super().__call__(*args, **kwargs) # 调用type的__call__方法 返回值为第一次创建的Spam对象
  9.             return self.__instance 
  10.         else:
  11.             return self.__instance
  13. # Example
  14. class Spam(metaclass=Singleton):
  15.     def __init__(self):
  16.         print('Creating Spam')
  17. """>>> a = Spam()
  18. Creating Spam
  19. >>> b = Spam()
  20. >>> a is b

不用元类的写法

  1. class _Spam:
  2.     def __init__(self):
  3.         print('Creating Spam')
  5. _spam_instance = None
  7. def Spam():
  8.     global _spam_instance
  10.     if _spam_instance is not None:
  11.         return _spam_instance
  12.     else:
  13.         _spam_instance = _Spam()
  14.         return _spam_instance