code: def retrieve_author(id=None, username=None):
      
  
      if id is None and username is None:
          raise SpigotAuthorException("Unable to retrieve an Author without an Identifier")
  
      if id is None:
          return SpigotAuthor.from_username(username)
      else:
          return SpigotAuthor.from_id(id)
docs: Retrieve a SpigotAuthor via their id, or username.
      :param id:
      :param username:
      :return: