I've been experimenting with MQTT in microPython. I have clients that publish & subscribe.
Regarding subscription I've created 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; it still returns None.
What are the other possible returns are possible by return_msg = client.check_msg() beside None?