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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_cache_key(user_or_username, size, prefix):
 """ Returns a cache key consisten of a username and image size. """

    if isinstance(user_or_username, get_user_model()):
        user_or_username = get_username(user_or_username)
    key = six.u('%s_%s_%s') % (prefix, user_or_username, size)
    return six.u('%s_%s') % (slugify(key)[:100],
                             hashlib.md5(force_bytes(key)).hexdigest())