def seed_dict(source_data, data_dictionary, seed):
    """Seed a dictionary with source data.
    
    Goes through a list of items and adds items not currently in
    data_dictionary to the dictionary. Added items are seeded with the passed
    seed. Updated dictionary is returned.
    
    Args:
        soure_data (list): List of items to add to dictionary (if not present).
        data_dictionary (dict): Dictionary to be updated.
        seed (str or int): Seed value to be added to new dictionary items.
    
    Returns:
        data_dictionary (dict): Updated dictionary.
    """
    # Get unique items in source_data)
    unique = set(source_data)
    unique_items = list(unique)
    # Find missing values and add to dictionary
    for item in unique_items:
        if item not in data_dictionary.keys():
            data_dictionary[item] = seed
    return data_dictionary