Is there a simple, elegant way to define singletons?

Python Programming

Question or problem about Python programming:

There seem to be many ways to define singletons in Python. Is there a consensus opinion on Stack Overflow?

How to solve the problem:

Solution 1:

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.

Solution 2:

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:

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.

            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)

Solution 3:

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"

Solution 4:

A slightly different approach to implement the singleton in Python is the borg pattern by Alex Martelli (Google employee and Python genius).

class Borg:
    __shared_state = {}
    def __init__(self):
        self.__dict__ = self.__shared_state

So instead of forcing all instances to have the same identity, they share state.

Solution 5:

The module approach works well. If I absolutely need a singleton I prefer the Metaclass approach.

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

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

class MyClass(object):
    __metaclass__ = Singleton

Solution 6:

See this implementation from PEP318, implementing the singleton pattern with a decorator:

def singleton(cls):
    instances = {}
    def getinstance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]
    return getinstance

class MyClass:

Solution 7:

As the accepted answer says, the most idiomatic way is to just use a module.

With that in mind, here’s a proof of concept:

def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
        del cls.__init__
    except AttributeError:
    return cls

See the Python data model for more details on __new__.


class Duck(object):

if Duck() is Duck():
    print "It works!"
    print "It doesn't work!"


  1. You have to use new-style classes (derive from object) for this.

  2. The singleton is initialized when it is defined, rather than the first time it’s used.

  3. This is just a toy example. I’ve never actually used this in production code, and don’t plan to.

Solution 8:

The Python documentation does cover this:

class Singleton(object):
    def __new__(cls, *args, **kwds):
        it = cls.__dict__.get("__it__")
        if it is not None:
            return it
        cls.__it__ = it = object.__new__(cls)
        it.init(*args, **kwds)
        return it
    def init(self, *args, **kwds):

I would probably rewrite it to look more like this:

class Singleton(object):
    """Use to create a singleton"""
    def __new__(cls, *args, **kwds):
        >>> s = Singleton()
        >>> p = Singleton()
        >>> id(s) == id(p)
        self = "__self__"
        if not hasattr(cls, self):
            instance = object.__new__(cls)
            instance.init(*args, **kwds)
            setattr(cls, self, instance)
        return getattr(cls, self)

    def init(self, *args, **kwds):

It should be relatively clean to extend this:

class Bus(Singleton):
    def init(self, label=None, *args, **kwds):
        self.label = label
        self.channels = [Channel("system"), Channel("app")]

Solution 9:

I’m very unsure about this, but my project uses ‘convention singletons’ (not enforced singletons), that is, if I have a class called DataController, I define this in the same module:

_data_controller = None
def GetDataController():
    global _data_controller
    if _data_controller is None:
        _data_controller = DataController()
    return _data_controller

It is not elegant, since it’s a full six lines. But all my singletons use this pattern, and it’s at least very explicit (which is pythonic).

Solution 10:

The one time I wrote a singleton in Python I used a class where all the member functions had the classmethod decorator.

class foo:
  x = 1

  def increment(cls, y = 1):
    cls.x += y

Hope this helps!