Singleton- python, Situation with the need of only one object

Question

I am having a situation where I have to make sure I only have one object. I am thinking about implementing singleton like following:

class One_Of_a_Kind:
    def __init__(self):
        self.do_some_setup()


class One_Creator:
    Only_One = None
    def __new__(cls, *args, **kwargs):
        if One_Creator.Only_One:
            return One_Creator.Only_One
        else:
            One_Creator.Only_One = One_of_a_Kind()
            return One_Creator.Only_One

Since I am reading a lot about singleton (pros and cons), I am little hesitant in implementing this code. I would like to know if it is okay and/or considered good practice to use in a situation where only one object of certain class in needed ( or mandatory).

Is there a better way of implementing the same thing?

Answer 1

A better way is the Borg design pattern. It's very simple to implement in python:

class Borg:

    _shared_state = {}

    def __init__(self):
        self.__dict__ = self._shared_state

You don't actually have a single instance, but each instance shares the same state - which is the part that matters.

Answer 2

When and how to use a singleton is a pretty broad question and primarily opinion based. That being said, I would implement it like this:

class Singleton(object):
    state = {}
    def __init__(self):
        self.__dict__ = Singleton.state

The idea is taken from here. This is sort of a pseudo-singleton, as two instance will not share the same id:

>>> s1 = Singleton()
>>> s2 = Singleton()
>>> s1 is s2
False
>>> s1.x = 42
>>> s2.x
42