@classmethod
def get_or_create(cls, file_path=None):
    """
        Returns the instance of the class (representing the singleton)
        or instantiate it if no instance already exists.
        If `file_path` is provided, starts to read it.
        """
    if cls._instance is None:
        cls._instance = cls(file_path)
    elif file_path is not None:
        cls._instance.open_file(file_path)
    return cls._instance
