turns class to singleton
    :param cls: class itself
    :return: function that either creates new instance of the class or returns existing one

def singleton(cls):
    """
    turns class to singleton
    :param cls: class itself
    :return: function that either creates new instance of the class or returns existing one
    """

    # the only way to implement nonlocal closure variables in Python 2.X
    instances = {}

    def get_instance():
        if cls not in instances:
            instances[cls] = cls()
        return instances[cls]

    return get_instance