为什么“ time.sleep”不延迟MQTT发布消息？

Question

Python 2.7

I want to publish 3 times and interval=3 sec.

So I try to use time.sleep(3) , then publish.

My code is like:

for i in range(3):
    print(i)
    mqttc.publish("test", "hello")
    time.sleep(3)

The result should be like:

0
(Publish)
(delay 3 sec)
1
(Publish)
(delay 3 sec)
2
(Publish)
(delay 3 sec)

But real result is:

0
(delay 3 sec)
1
(delay 3 sec)
2
(delay 3 sec)
(Publish)
(Publish)
(Publish)

The real result is found from MQTT.fx and Python subscribe.

Delay is normal worked to "Print", but "Publish" not,

I don't understand why publish is continuous...

Answer 1

It's quite simple: MQTT needs to have its event loop running to process network communications. By sleeping the thread you effectively wrestle control away from MQTT and keep it from doing anything useful.

Instead of blocking the thread, you should let MQTT's event loop handle timing:

for i in range(3):
    print(i)
    mqttc.publish("test", "hello")
    mqttc.loop(timeout=3.0)

You can also run a background event loop on a different thread, using loop_start() and loop_stop() but threads are not a cheap resource so if you'd be instead wasting a whole thread on doing literally nothing, it's better to let that thread be useful instead. The rule of thumb with threads is that if with one thread you had one problem, with two threads you have two or more problems (they tend to multiply).