Singleton implementations

singleton.py

import functools

def singleton(cls, *args, **kwargs):
    instances = {}

    @functools.wraps(cls)
    def wrapper():
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return wrapper

singleton2.py

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

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

class A(object):
    __metaclass__ = Singleton

    def __init__(self):
        self.xx = 1