[Python] Does on_message in paho mqtt run in a new thread?

Lets assume I have subscribed to some topic and the mosquitto server is continuosly publishing some message on that topic.

How does the on_message method get invoked when a new message arrives from mosquitto broker? Does it run on the main thread thereby blocking it and only processing the next message when the current one is processed or does it spawn a new thread with the on_message method every time a new message arrives?

import paho.mqtt.client as mqtt

def on_message(client, userdata, message):
print message.payload
return True

topics = ["topic1",]
client = mqtt.Client("testclient",protocol=mqtt.MQTTv31,clean_session=True)
client.on_message=on_message
client.connect("127.0.0.1", 1883, 60)
for tpc in topics:
client.subscribe(tpc,0)
client.loop_forever()

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