是否有一种简单,优雅的方式来定义单身人士？

Question

似乎有很多方法可以在Python中定义单例.Stack Overflow是否有共识？

Answer 1

我并不真正看到需要,因为具有函数(而不是类)的模块可以很好地用作单例.它的所有变量都将绑定到模块,无论如何都无法重复实例化.

如果您确实希望使用类,则无法在Python中创建私有类或私有构造函数,因此除了通过使用API​​的约定之外,您无法防止多个实例化.我仍然只是将方法放在一个模块中,并将模块视为单例.

Answer 2

这是我自己实现的单身人士.你所要做的就是装饰课程; 要获得单例,您必须使用该Instance方法.这是一个例子:

@Singleton
class Foo:
   def __init__(self):
       print 'Foo created'

f = Foo() # Error, this isn't how you get the instance of a singleton

f = Foo.instance() # Good. Being explicit is in line with the Python Zen
g = Foo.instance() # Returns already created instance

print f is g # True

这是代码:

class Singleton:
    """
    A non-thread-safe helper class to ease implementing singletons.
    This should be used as a decorator -- not a metaclass -- to the
    class that should be a singleton.

    The decorated class can define one `__init__` function that
    takes only the `self` argument. Also, the decorated class cannot be
    inherited from. Other than that, there are no restrictions that apply
    to the decorated class.

    To get the singleton instance, use the `instance` method. Trying
    to use `__call__` will result in a `TypeError` being raised.

    """

    def __init__(self, decorated):
        self._decorated = decorated

    def instance(self):
        """
        Returns the singleton instance. Upon its first call, it creates a
        new instance of the decorated class and calls its `__init__` method.
        On all subsequent calls, the already created instance is returned.

        """
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError('Singletons must be accessed through `instance()`.')

    def __instancecheck__(self, inst):
        return isinstance(inst, self._decorated)

Answer 3

您可以__new__像这样覆盖方法:

class Singleton(object):
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(
                                cls, *args, **kwargs)
        return cls._instance


if __name__ == '__main__':
    s1 = Singleton()
    s2 = Singleton()
    if (id(s1) == id(s2)):
        print "Same"
    else:
        print "Different"

Answer 4

在Python中实现单例的略微不同的方法是Alex Martelli(Google员工和Python天才)的borg模式.

class Borg:
    __shared_state = {}
    def __init__(self):
        self.__dict__ = self.__shared_state

因此,它们不是强制所有实例具有相同的身份,而是共享状态.

Answer 5

模块方法运作良好.如果我绝对需要单身人士,我更喜欢Metaclass方法.

class Singleton(type):
    def __init__(cls, name, bases, dict):
        super(Singleton, cls).__init__(name, bases, dict)
        cls.instance = None 

    def __call__(cls,*args,**kw):
        if cls.instance is None:
            cls.instance = super(Singleton, cls).__call__(*args, **kw)
        return cls.instance

class MyClass(object):
    __metaclass__ = Singleton

Answer 6

从PEP318看到这个实现,用装饰器实现单例模式:

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

@singleton
class MyClass:
    ...

Answer 7

正如公认的答案所说,最惯用的方法是使用模块.

考虑到这一点,这是一个概念证明:

def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
    try:
        del cls.__init__
    except AttributeError:
        pass
    return cls

有关更多详细信息,请参阅Python数据模型__new__.

例:

@singleton
class Duck(object):
    pass

if Duck() is Duck():
    print "It works!"
else:
    print "It doesn't work!"

笔记:

1. 您必须为此使用新式类(派生自object).

2. 单例在定义时初始化,而不是在第一次使用时初始化.

3. 这只是一个玩具的例子.我从来没有在生产代码中实际使用它,也不打算这样做.

Answer 8

我对此非常不确定,但我的项目使用'convention singletons'(非强制单例),也就是说,如果我有一个叫做的类DataController,我在同一个模块中定义它:

_data_controller = None
def GetDataController():
    global _data_controller
    if _data_controller is None:
        _data_controller = DataController()
    return _data_controller

它不优雅,因为它是完整的六行.但我所有的单身人士都使用这种模式,至少非常明确(这是pythonic).

Answer 9

在Python文档确实涉及到这一点:

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

我可能会重写它看起来更像这样:

class Singleton(object):
    """Use to create a singleton"""
    def __new__(cls, *args, **kwds):
        """
        >>> s = Singleton()
        >>> p = Singleton()
        >>> id(s) == id(p)
        True
        """
        self = "__self__"
        if not hasattr(cls, self):
            instance = object.__new__(cls)
            instance.init(*args, **kwds)
            setattr(cls, self, instance)
        return getattr(cls, self)

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

扩展它应该相对干净:

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

Answer 10

有一次我在Python中编写了一个单例,我使用了一个类,其中所有成员函数都有classmethod装饰器.

class foo:
  x = 1

  @classmethod
  def increment(cls, y = 1):
    cls.x += y

Answer 11

如果要在未来装饰(注释)类,创建单例装饰器(也称为注释)是一种优雅的方式.然后你就把@singleton放在你的类定义之前.

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

@singleton
class MyClass:
    ...

Answer 12

谷歌测试博客上还有一些有趣的文章,讨论为什么单身人士/可能是坏的并且是一种反模式:

• 单身人士是病态的骗子
• 所有单身人士都去了哪里？
• 单身人士的根本原因

Answer 13

我认为强迫一个类或一个实例成为一个单例是过度的.就个人而言,我喜欢定义一个普通的可实例化类,一个半私有引用和一个简单的工厂函数.

class NothingSpecial:
    pass

_the_one_and_only = None

def TheOneAndOnly():
    global _the_one_and_only
    if not _the_one_and_only:
        _the_one_and_only = NothingSpecial()
    return _the_one_and_only

或者,如果在首次导入模块时实例化没有问题:

class NothingSpecial:
    pass

THE_ONE_AND_ONLY = NothingSpecial()

这样,您可以针对没有副作用的新实例编写测试,并且不需要在模块中添加全局语句,如果需要,您可以在将来派生变体.