Demo on how to define Singleton Classes on Python

singleton_class.py

#! /usr/bin/python

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)

        return cls._instances[cls]


class ExampleClass(object):
    __metaclass__ = Singleton

    def __init__(self):
        self.name = 'World'

    def set_name(self, name):
        self.name = name

    def say_hello(self):
        print 'Hello, {0}!'.format(self.name)

if __name__ == '__main__':
    print '{0:=^40}'.format(' Create Both Objects ')
    obj1 = ExampleClass()
    obj2 = ExampleClass()

    print '{0:~^40}'.format(' Say Hello before setting any name. ')
    obj1.say_hello()
    obj2.say_hello()

    print '{0:~^40}'.format(' Set Name on Obj 1 to "Earth" ')
    obj1.set_name('Earth')

    print '{0:~^40}'.format(' Say Hello only on Obj 2 ')
    obj2.say_hello()

    print '{0:~^40}'.format(' Set Name on Obj 2 to "Moon"')
    obj2.set_name('Moon')

    print '{0:~^40}'.format(' Say Hello only on Obj 1 ')
    obj1.say_hello()