python中如何实现单例模式

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python中实现Singleton模式

主要参考来自于stackoverflow

方法一：装饰器decorator

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def singleton(cls):
    instances = {}
	def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance
@singleton
class MyClass:
    ...

这个例子来自PEP 318 - Decorators for Functions and Methods

方法二：基类Base class

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class Singleton(object):
  _instance = None
  def __new__(class_, *args, **kwargs):
    if not isinstance(class_._instance, class_):
        class_._instance = object.__new__(class_, *args, **kwargs)
    return class_._instance
class MyClass(Singleton, BaseClass):
  pass

方法三：元类metaclass

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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]
#Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton
#Python3
class MyClass(BaseClass, metaclass=Singleton):
    pass

方法四：用装饰器返回一个相同名字的类

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def singleton(class_):
  class class_w(class_):
    _instance = None
    def __new__(class_, *args, **kwargs):
      if class_w._instance is None:
          class_w._instance = super(class_w, 
                                    class_).__new__(class_, 
                                                    *args, 
                                                    **kwargs)
          class_w._instance._sealed = False
      return class_w._instance
    def __init__(self, *args, **kwargs):
      if self._sealed:
        return
      super(class_w, self).__init__(*args, **kwargs)
      self._sealed = True
  class_w.__name__ = class_.__name__
  return class_w
@singleton
class MyClass(BaseClass):
    pass

其他的思考

1、利用模块来代替单例的实现，将模块看成一个Singleton，直接在里面定义方法，参考这里
这可能是the most idiomatic way。

2、Borg(共享)模式

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class Borg:
    __shared_state = {}
    def __init__(self):
        self.__dict__ = self.__shared_state
    # and whatever else you want in your class -- that's all!

更多讨论参考这里