有没有一种简单、优雅的方法来定义单例？

Question

在Python中定义singletons的方法似乎很多。对于堆栈溢出是否有共识？

Answer 1

我并不认为有必要，因为一个带有函数（而不是类）的模块可以很好地充当单例。它的所有变量都将绑定到模块，但无论如何不能重复实例化该模块。

如果您确实希望使用一个类，那么在Python中无法创建私有类或私有构造函数，因此除了通过使用API中的约定之外，您不能针对多个实例化进行保护。我仍然会将方法放在一个模块中，并将该模块视为单例。

Answer 2

您可以重写__new__方法，如下所示：

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 3

这是我自己实现的singleton。您所要做的就是修饰类；要获得singleton，您就必须使用Instance方法。下面是一个例子：

@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

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)