The most feature complete MQTT client for Python is Mosquitto. Below you see an sample app which both publish and subscribes to CloudAMQP MQTT.

CloudAMQP_MQTT_URL Structure mqtt://cloudamqp_username:cloudamqp_password@hostname:port

import mosquitto, os, urlparse

# Define event callbacks
def on_connect(mosq, obj, rc):
    print("rc: " + str(rc))

def on_message(mosq, obj, msg):
    print(msg.topic + " " + str(msg.qos) + " " + str(msg.payload))

def on_publish(mosq, obj, mid):
    print("mid: " + str(mid))

def on_subscribe(mosq, obj, mid, granted_qos):
    print("Subscribed: " + str(mid) + " " + str(granted_qos))

def on_log(mosq, obj, level, string):
    print(string)

mqttc = mosquitto.Mosquitto()
# Assign event callbacks
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe

# Uncomment to enable debug messages
#mqttc.on_log = on_log

# Parse CLOUDAMQP_MQTT_URL (or fallback to localhost)
url_str = os.environ.get('CLOUDAMQP_MQTT_URL', 'mqtt://localhost:1883')
url = urlparse.urlparse(url_str)

# Connect
mqttc.username_pw_set(url.username, url.password)
mqttc.connect(url.hostname, url.port)

# Start subscribe, with QoS level 0
mqttc.subscribe("hello/world", 0)

# Publish a message
mqttc.publish("hello/world", "my message")

# Continue the network loop, exit when an error occurs
rc = 0
while rc == 0:
    rc = mqttc.loop()
print("rc: " + str(rc))

The full code can be seen at github.com/CloudMQTT/python-mqtt-example.

Alternative clients

Another good MQTT client for Python is paho-mqtt 1.0. The Paho Python Client class provides some helper functions to make publishing one off messages to an MQTT server very straightforward. Futher documentation can be found here.