singleton.py

from threading import Lock

# ala https://en.wikipedia.org/wiki/Singleton_pattern#Python_implementation
# and https://stackoverflow.com/questions/51896862/how-to-create-singleton-class-with-arguments-in-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]

# Based on tornado.ioloop.IOLoop.instance() approach.
# See https://github.com/facebook/tornado
# Whole idea for this metaclass is taken from: https://stackoverflow.com/a/6798042/2402281
class ThreadSafeSingleton(type):
    _instances = {}
    _singleton_lock = Lock()

    def __call__(cls, *args, **kwargs):
        # double-checked locking pattern (https://en.wikipedia.org/wiki/Double-checked_locking)
        if cls not in cls._instances:
            with cls._singleton_lock:
                if cls not in cls._instances:
                    cls._instances[cls] = super(ThreadSafeSingleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]