code: def get_cache_key(user_or_username, size, prefix):
      
      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())
docs: Returns a cache key consisten of a username and image size.