I've been experimenting with MQTT in microPython. I have clients that publish & subscribe. Regarding subscription I've used examples that use client.check_msg() and return_msg = client.check_msg() in a loop; retrieving messages in a previously set call-back function.
What's not clear is the form return_msg = client.check_msg(). So far whether the call-back retrieves a message or not the return_msg always appears to return None (a nonetype). I would have though it might also return the received message, but apparently it does not.
What are the other possible returns are possible by return_msg = client.check_msg() beside None?