实现单例的四种实现方法

# <单例>
# Singleton
#
# 单例：即单个实例，指的是同一个类实例化多次的结果指向同一个对象，用于节省内存空间
# 如果我们从配置文件中读取配置来进行实例化，在配置相同的情况下，就没必要重复产生对象浪费内存了
#
#     实现原理，为了节省空间，结合需求让同一个类多次实例化后结果指向同一个对象
#     可理解为开关，一次进入后更改 instance的状态

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<< 1.类方法实现 >>

class Singleton:
    __instance = None
    def change_instance(cls):
        if not cls.__instance:
            cls.__instance = cls()
        return cls.__instance

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<< 2.装饰器实现 >>

def outer(fn):
    _instance = None
    def inner(*args, **kwargs):
        nonlocal _instance
        if _instance == None:
            _instance = fn(*args, **kwargs)
        return _instance
    return inner

@outer
class Music:
    pass

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<< 3.元类方法实现 >>

class SingleMeta(type):
    __instance = None
    def __call__(cls, *args, **kwargs):
        if SingleMeta.__instance == None:
            SingleMeta.__instance = object.__new__(cls) # {}
            cls.__init__(SingleMeta.__instance, *args, **kwargs)
            print(SingleMeta.__instance.__dict__) # {} {'name': 'egon'}
        return SingleMeta.__instance

class Songs(metaclass=SingleMeta):
    def __init__(self):
        self.name = 'egon'
    pass

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<< 4.反射方法实现 >>

class Singleton(object):
  def __new__(cls):
    if not hasattr(cls, 'instance'):
      cls.instance = super().__new__(cls)
    return cls.instance

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<< 5.__new__方法实现 >>

class Singleton():
   __obj = None
   def __new__(cls):
      if cls.__obj is None:
         cls.__obj = object.__new__(cls)
      return cls.__obj