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JZ02.单例模式

描述

设计一个类，只能生成该类的一个实例

思路

• 函数装饰器

  def singleton(cls):
      _instance = {}
    
      def inner():
          if cls not in _instance:
              _instance[cls] = cls()
          return _instance[cls]
      return inner
    
    
  @singleton
  class Cls:
      def __init__(self):
          pass
    
    
  cls1 = Cls()
  cls2 = Cls()
  print(id(cls1) == id(cls2))
  

• 类装饰器

  class Singleton:
      def __init__(self, cls):
          self._cls = cls
          self._instance = {}
    
      def __call__(self, *args, **kwargs):
          if self._cls not in self._instance:
              self._instance[self._cls] = self._cls()
          return self._instance[self._cls]
    
    
  @Singleton
  class Cls:
      def __init__(self):
          pass
    
    
  cls1 = Cls()
  cls2 = Cls()
  print(id(cls1) == id(cls2))
  

• __new__ 关键字

  元类(metaclass) 可以通过方法 __metaclass__ 创造了类(class)，而类(class)通过方法 __new__ 创造了实例(instance)

  class Singleton:
      _instance = None
    
      def __new__(cls, *args, **kwargs):
          if cls._instance is None:
              cls._instance = object.__new__(cls, *args, **kwargs)
          return cls._instance
    
      def __init__(self):
          pass
    
    
  cls1 = Singleton()
  cls2 = Singleton()
  print(id(cls1) == id(cls2))
  

• __metaclass__

  使用 type 创造类

  def func(self):
      print("do sth")
        
  C = type("C", (), {"func": func})
  c = C()
  c.func()
  

  mataclass 实现单例

  class Singleton(type):
      _instances = {}
      def __call__(cls, *args, **kwargs):
          if cls not in cls._instances:
              cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
          return cls._instances[cls]
    
  class Cls4(metaclass=Singleton):
      pass
    
  cls1 = Cls4()
  cls2 = Cls4()
  print(id(cls1) == id(cls2))
  

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