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import threading


# Based on tornado.ioloop.IOLoop.instance() approach.
# See https://github.com/facebook/tornado
class SingletonMixin(object):
	__singleton_lock = threading.Lock()
	__singleton_instance = None

	@classmethod
	def instance(cls):
		if not cls.__singleton_instance:
			with cls.__singleton_lock:
				if not cls.__singleton_instance:
					cls.__singleton_instance = cls()
		return cls.__singleton_instance


if __name__ == '__main__':
	class A(SingletonMixin):
		pass

	class B(SingletonMixin):
		pass

	a, a2 = A.instance(), A.instance()
	b, b2 = B.instance(), B.instance()

	assert a is a2
	assert b is b2
	assert a is not b

	print('a:  %s\na2: %s' % (a, a2))
	print('b:  %s\nb2: %s' % (b, b2))