python 3 paho mqtt发布/订阅的json消息将不解析

新人在这里。

我有一个简单的python代码,它应该订阅一个主题,并使用mqtt协议将json负载发布到同一个主题。但由于某些原因,我无法将负载加载为json!

我在这里做错什么了?

# -*- coding: utf-8 -*-
import paho.mqtt.client as mqtt
import json
mqtt_broker      = '192.168.1.111'
mqtt_topic_one   = 'mqtt_topic/tops_one'
mqtt_topic_two   = 'mqtt_topic/tops_two'

json_data_1      = '''{
    "this_json": "info",
    "data": {
        "multi_keyval": {
            "1": "1",
            "5": "5",
            "15": "15"
        },
        "single_keyval": {
            "single_key": "200"
        }
    }
}'''


def pass_to_func_and_pub(data_to_pub):
    print(data_to_pub)                # --------> This PRINTS
    print(json.loads(data_to_pub))    # --------> This DOES NOT PRINT

    # The following two lines don't work either.
    unpacked_json = json.loads(data_to_pub)
    client.publish(mqtt_topic_two, unpacked_json['this_json'])


def on_connect(client, userdata, flags, rc):
    client.subscribe(mqtt_topic_one)
    client.publish(mqtt_topic_one, json_data_1)


def on_message(client, userdata, msg):
    pass_to_func_and_pub(str(msg.payload))

client = mqtt.Client()

client.on_connect = on_connect
client.on_message = on_message

client.connect(mqtt_broker)
try:
    client.loop_forever()
except KeyboardInterrupt:
    client.disconnect()
    print('MQTT client disconnected, exiting now.')