Python 用__new__方法实现单例的操作

class A:
 instance = None
 def __new__(cls, *args, **kwargs):
  if cls.instance is None:
   cls.instance = super().__new__(cls)
  return cls.instance

class A:
 instance = None
 init_flag = False # 初始化标记

 def __new__(cls, *args, **kwargs):
  if cls.instance is None:
   cls.instance = super().__new__(cls)
  return cls.instance

 def __init__(self):
  if A.init_flag:
   return
  print('执行了初始化方法')
  A.init_flag = True

if __name__ == '__main__':
 a = A()
 b = A()
 print(a)
 print(b)

# 饿汉式
class Singleton(object):
 # 重写创建实例的__new__方法
 def __new__(cls):
  # 如果类没有实例属性，进行实例化，否则返回实例
  if not hasattr(cls, 'instance'):
   cls.instance = super(Singleton, cls).__new__(cls)
  return cls.instance

# 懒汉式
class Singleton(object):
 __instance = None
 def __init__(self):
  if not self.__instance:
   print('调用__init__， 实例未创建')
  else:
   print('调用__init__，实例已经创建过了:', __instance)

 @classmethod
 def get_instance(cls):
  # 调用get_instance类方法的时候才会生成Singleton实例
  if not cls.__instance:
   cls.__instance = Singleton()
  return cls.__instance