class SingletonMixin:
    """ Threading safe singleton mixin class.  Will run __init__ on subclasses for every
    invocation, so check self._initialized from __init__ to get around this if not desired.

    Use:
    class MyClass(SingletonMixin):
        def __init__(self):
          if not self._initialized:
             # initialize here
             self.foo = 'bar'
             # when done, set initialized to True
             self._initialized = True
    """
    _instances: dict[type, Any] = {}
    _lock = threading.Lock()
    _initialized: bool

    def __new__(cls):
        with SingletonMixin._lock:
            if cls not in SingletonMixin._instances:
                # Create instance
                instance = super().__new__(cls)
                SingletonMixin._instances[cls] = instance
                instance._initialized = False
        # Return existing instance
        return SingletonMixin._instances[cls]