有没有一种简单、优雅的方式来定义单例？

Question

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

原文由 Jamie 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

我真的不认为有必要，因为具有功能的模块（而不是类）可以很好地用作单例。它的所有变量都将绑定到模块，无论如何都不能重复实例化。

如果您确实希望使用一个类，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，您无法防止多次实例化。我仍然只是将方法放在一个模块中，并将该模块视为单例。

原文由 Staale 发布，翻译遵循 CC BY-SA 3.0 许可协议

Answer 2

这是我自己的单例实现。您所要做的就是装饰班级；要获得单身人士，您必须使用 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)

原文由 Paul Manta 发布，翻译遵循 CC BY-SA 3.0 许可协议