class Singleton: """ (From Stackoverflow : http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python) 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. 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. Limitations: The decorated class cannot be inherited from. """ 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)