"""Implementation of generic singleton object"""

from threading import Lock


class SingletonMeta(type):
    """
    Thread-safe implementation of Singleton.
    """

    _instances = {}

    _lock = Lock()
    """
    Synchronizes threads during first access to the Singleton.
    """

    def __call__(cls, *args, **kwargs):
        """
        Possible changes to the value of the `__init__` argument do not affect
        the returned instance.
        """
        # First thread acquires the lock
        with cls._lock:
            if cls not in cls._instances:
                instance = super().__call__(*args, **kwargs)
                cls._instances[cls] = instance
        return cls._instances[cls]