def combine_histories(existing_history: dict, new_history: dict) -> dict:
    """Combines model training history objects, such that the new history from a more recent training session is
    appended to the existing history from a previous training session.

    Args:
        existing_history: Model training history from a previous training session.
        new_history: Model training history from a more recent training session.

    Returns:
        Combined model training history.
    """
    combined_history = existing_history.copy()
    for key, value in new_history.items():
        if type(value) is list:
            # Note:  this avoids combining attributes like model_name and is_model_trained
            combined_history.setdefault(key, list()).extend(value)
    return combined_history