Instantiate Album or Image instances not already in cache.

    :param dict album_cache: Cache of Imgur albums to update. Keys are Imgur IDs, values are Album instances.
    :param dict image_cache: Cache of Imgur images to update. Keys are Imgur IDs, values are Image instances.
    :param iter albums: List of album Imgur IDs.
    :param iter images: List of image Imgur IDs.

    :return: Same album and image cache dictionaries from parameters.
    :rtype: tuple

def initialize(album_cache, image_cache, albums, images):
    """Instantiate Album or Image instances not already in cache.

    :param dict album_cache: Cache of Imgur albums to update. Keys are Imgur IDs, values are Album instances.
    :param dict image_cache: Cache of Imgur images to update. Keys are Imgur IDs, values are Image instances.
    :param iter albums: List of album Imgur IDs.
    :param iter images: List of image Imgur IDs.

    :return: Same album and image cache dictionaries from parameters.
    :rtype: tuple
    """
    if not hasattr(album_cache, 'setdefault'):
        album_cache = dict()
    if not hasattr(image_cache, 'setdefault'):
        image_cache = dict()
    for imgur_id in albums:
        album_cache.setdefault(imgur_id, Album(imgur_id))
    for imgur_id in images:
        image_cache.setdefault(imgur_id, Image(imgur_id))
    return album_cache, image_cache