How to make sure queue is empty before exiting main thread

The comments touch on using .join(), but depending on your use case, using a join may make threading pointless.

I assume that your main thread will be doing things other than adding items to the queue - and may be shut down at any point, you just want to ensure that your queue is empty before shutting down is complete.

At the end of your main thread, you could add a simple empty check in a loop.

while not q.empty():
    sleep(1)

If you don't set t.daemon = True then the thread will never finish. Setting the thread as a daemon thread will mean that the thread does not cause your program to stay running when the main thread finishes.

Put a special item (e.g. None) in the queue, that signals the worker thread to stop.

import queue
import threading

q = queue.Queue()

def _worker():
  while True:
      msg = q.get(block=True)
      if msg is None:
          return
      print(msg) # do your stuff here

t = threading.Thread(target=_worker)
#t.daemon = True
t.start()

q.put('asdf-1')
q.put('asdf-2')
q.put('asdf-4')
q.put('asdf-4')
q.put(None)
t.join()