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")]
        ...

这是我自己对单例对象的实现。你所要做的就是布置课堂;为了获得单例，你必须使用Instance方法。这里有一个例子:

@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

下面是代码:

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)

模块方法工作得很好。如果我绝对需要一个单例，我更喜欢元类方法。

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

我真的不认为有这个需要，因为一个带有函数(而不是类)的模块可以很好地作为一个单例。它的所有变量都将绑定到模块，无论如何都不能重复实例化。

如果你确实希望使用一个类，在Python中没有办法创建私有类或私有构造函数，所以你不能防止多个实例化，只能通过使用API中的约定。我仍然会将方法放在模块中，并将模块视为单例模块。

你可以像这样重写__new__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"

由ActiveState提供的Python实现的单例模式。

看起来窍门是把应该只有一个实例的类放在另一个类中。