def call_on_class_only(*args, **kwargs):
    """Helper to raise `AttributeError` if a class method is called on an instance. Used internally.

    Parameters
    ----------
    *args
        Variable length argument list.
    **kwargs
        Arbitrary keyword arguments.

    Raises
    ------
    AttributeError
        If a class method is called on an instance.

    """
    raise AttributeError('This method should be called on a class object.')
