Is there a simple, elegant way to define singletons?

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.

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.

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

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

There are also some interesting articles on the Google Testing blog, discussing why singleton are/may be bad and are an anti-pattern:

• Singletons are Pathological Liars
• Where Have All the Singletons Gone?
• Root Cause of Singletons

The Singleton Pattern implemented with Python courtesy of ActiveState.

It looks like the trick is to put the class that's supposed to only have one instance inside of another class.

class Singleton(object[,...]):

    staticVar1 = None
    staticVar2 = None

    def __init__(self):
        if self.__class__.staticVar1==None :
            # create class instance variable for instantiation of class
            # assign class instance variable values to class static variables
        else:
            # assign class static variable values to class instance variables

Being relatively new to Python I'm not sure what the most common idiom is, but the simplest thing I can think of is just using a module instead of a class. What would have been instance methods on your class become just functions in the module and any data just becomes variables in the module instead of members of the class. I suspect this is the pythonic approach to solving the type of problem that people use singletons for.

If you really want a singleton class, there's a reasonable implementation described on the first hit on Google for "Python singleton", specifically:

class Singleton:
    __single = None
    def __init__( self ):
        if Singleton.__single:
            raise Singleton.__single
        Singleton.__single = self

That seems to do the trick.