docs: def update_feature_value(host_name, client_name, client_pass, user_twitter_id, feature_name, feature_score):
      """
      Updates a single topic score, for a single user.
  
      Inputs: - host_name: A string containing the address of the machine where the PServer instance is hosted.
              - client_name: The PServer client name.
              - client_pass: The PServer client's password.
              - user_twitter_id: A Twitter user identifier.
              - feature_name: A specific PServer feature name.
              - feature_score: The corresponding score.
      """
      username = str(user_twitter_id)
      feature_value = "{0:.2f}".format(feature_score)
      joined_ftr_value = "ftr_" + feature_name + "=" + str(feature_value)
      values = "usr=%s&%s" % (username, joined_ftr_value)
  
      # Construct request.
      request = construct_request(model_type="pers",
                                  client_name=client_name,
                                  client_pass=client_pass,
                                  command="setusr",
                                  values=values)
  
      # Send request.
      send_request(host_name,
                   request)