python – Is there a simple, elegant way to define singletons?

python – Is there a simple, elegant way to define singletons?

I dont 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 cant 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.

Heres 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. Heres an example:

class Foo:
   def __init__(self):
       print Foo created

f = Foo() # Error, this isnt 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 heres 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.

            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)

python – Is there a simple, elegant way to define singletons?

You can override the __new__ method like this:

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
        print Different

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