在Python中似乎有很多方法来定义单例对象。对Stack Overflow是否有一致的意见?


当前回答

class Singeltone(type):
    instances = dict()

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in Singeltone.instances:            
            Singeltone.instances[cls.__name__] = type.__call__(cls, *args, **kwargs)
        return Singeltone.instances[cls.__name__]


class Test(object):
    __metaclass__ = Singeltone


inst0 = Test()
inst1 = Test()
print(id(inst1) == id(inst0))

其他回答

如果你不想要上面的基于元类的解决方案,并且你不喜欢简单的基于函数装饰器的方法(例如,因为在这种情况下,单例类上的静态方法将不起作用),这种妥协是可行的:

class singleton(object):
  """Singleton decorator."""

  def __init__(self, cls):
      self.__dict__['cls'] = cls

  instances = {}

  def __call__(self):
      if self.cls not in self.instances:
          self.instances[self.cls] = self.cls()
      return self.instances[self.cls]

  def __getattr__(self, attr):
      return getattr(self.__dict__['cls'], attr)

  def __setattr__(self, attr, value):
      return setattr(self.__dict__['cls'], attr, value)

作为Python的新手,我不确定最常见的习语是什么,但我能想到的最简单的事情就是使用模块而不是类。类中的实例方法变成了模块中的函数,任何数据都变成了模块中的变量,而不是类的成员。我怀疑这是解决人们使用单例对象的问题的python方法。

如果你真的想要一个单例类,有一个合理的实现描述在谷歌的第一个点击“Python singleton”,具体地说:

class Singleton:
    __single = None
    def __init__( self ):
        if Singleton.__single:
            raise Singleton.__single
        Singleton.__single = self

这似乎奏效了。

如果你想继续装饰(注释)类,创建一个单例装饰器(又名注释)是一种优雅的方式。然后将@singleton放在类定义之前。

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

@singleton
class MyClass:
    ...

好吧,我知道,单胞胎可能是好的,也可能是坏的。这是我的实现,我只是扩展了一个经典的方法,在里面引入一个缓存,并产生许多不同类型的实例,或者许多相同类型的实例,但具有不同的参数。

我称它为Singleton_group,因为它将相似的实例分组在一起,并防止创建具有相同参数的相同类的对象:

# Peppelinux's cached singleton
class Singleton_group(object):
    __instances_args_dict = {}
    def __new__(cls, *args, **kwargs):
        if not cls.__instances_args_dict.get((cls.__name__, args, str(kwargs))):
            cls.__instances_args_dict[(cls.__name__, args, str(kwargs))] = super(Singleton_group, cls).__new__(cls, *args, **kwargs)
        return cls.__instances_args_dict.get((cls.__name__, args, str(kwargs)))


# It's a dummy real world use example:
class test(Singleton_group):
    def __init__(self, salute):
        self.salute = salute

a = test('bye')
b = test('hi')
c = test('bye')
d = test('hi')
e = test('goodbye')
f = test('goodbye')

id(a)
3070148780L

id(b)
3070148908L

id(c)
3070148780L

b == d
True


b._Singleton_group__instances_args_dict

{('test', ('bye',), '{}'): <__main__.test object at 0xb6fec0ac>,
 ('test', ('goodbye',), '{}'): <__main__.test object at 0xb6fec32c>,
 ('test', ('hi',), '{}'): <__main__.test object at 0xb6fec12c>}

每个对象都携带单例缓存…这可能是邪恶的,但对一些人来说很有用:)

Python文档涵盖了这些内容:

class Singleton(object):
    def __new__(cls, *args, **kwds):
        it = cls.__dict__.get("__it__")
        if it is not None:
            return it
        cls.__it__ = it = object.__new__(cls)
        it.init(*args, **kwds)
        return it
    def init(self, *args, **kwds):
        pass

我可能会重写成这样:

class Singleton(object):
    """Use to create a singleton"""
    def __new__(cls, *args, **kwds):
        """
        >>> s = Singleton()
        >>> p = Singleton()
        >>> id(s) == id(p)
        True
        """
        it_id = "__it__"
        # getattr will dip into base classes, so __dict__ must be used
        it = cls.__dict__.get(it_id, None)
        if it is not None:
            return it
        it = object.__new__(cls)
        setattr(cls, it_id, it)
        it.init(*args, **kwds)
        return it

    def init(self, *args, **kwds):
        pass


class A(Singleton):
    pass


class B(Singleton):
    pass


class C(A):
    pass


assert A() is A()
assert B() is B()
assert C() is C()
assert A() is not B()
assert C() is not B()
assert C() is not A()

它应该是相对干净的扩展:

class Bus(Singleton):
    def init(self, label=None, *args, **kwds):
        self.label = label
        self.channels = [Channel("system"), Channel("app")]
        ...