I don't really see the need, as a module with functions (and not a class) would serve well as a singleton. All its variables would be bound to the module, which could not be instantiated repeatedly anyway.
If you do wish to use a class, there is no way of creating private classes or private constructors in Python, so you can't protect against multiple instantiations, other than just via convention in use of your API. I would still just put methods in a module, and consider the module as the singleton.
Here's my own implementation of singletons. All you have to do is decorate the class; to get the singleton, you then have to use the
Instance method. Here's an example:
@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
And here's the code:
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)