def generate_fn(**params) -> str:
    """
    Generate a unique filename from given params.

    This function is used
    internally with the parameters used by `fetch_data()`.  It is provided here
    in case you need to generate the filename from a given set of params.

    Args:
        **params: params to consider.

    Returns:
        (str): Fn from params.
    """
    fn = "-".join(f"{k}_{x}" for k, x in params.items() if x is not None)
    return f"marsdata_{fn}.txt"