the _drop-method is used for unittesting. Easy way to get rid of all your old instances.

SingletonTypes.py

__author__ = 'Christoph Gerneth'

class SingletonType(type):
    def __call__(self, *args, **kwargs):
        try:
            return self.__instance
        except AttributeError:
            self.__instance = super(SingletonType, self).__call__(*args, **kwargs)
            return self.__instance

    def _drop(self):
         "Drop the instance (for testing purposes)."
         del self.__instance

how to use the singleton instance:

class SingletonObject(object):
    __metaclass__ = SingletonType

and how to use them in unittests:

import unittest
class SingletonTypeTestCase(unittest.TestCase):
    def test_something(self):
        [SingletonObject() for i in range(10)]
    def tearDown(self):
        SingletonObject._drop()