Python threading.Event() - Ensuring all waiting threads wake up on event.set()

Question

I have a number of threads which wait on an event, perform some action, then wait on the event again. Another thread will trigger the event when it\'s appropriate.

I can

Answer 1

You don't need an Event, and you don't need both a Lock and a Queue. All you need is a Queue.

Call queue.put to drop a message in without waiting for it to be delivered or processed.

Call queue.get in the worker thread to wait for a message to arrive.

import threading
import Queue

active_queues = []

class Worker(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)
        self.mailbox = Queue.Queue()
        active_queues.append(self.mailbox)

    def run(self):
        while True:
            data = self.mailbox.get()
            if data == 'shutdown':
                print self, 'shutting down'
                return
            print self, 'received a message:', data

    def stop(self):
        active_queues.remove(self.mailbox)
        self.mailbox.put("shutdown")
        self.join()


def broadcast_event(data):
    for q in active_queues:
        q.put(data)

t1 = Worker()
t2 = Worker()
t1.start()
t2.start()
broadcast_event("first event")
broadcast_event("second event")
broadcast_event("shutdown")

t1.stop()
t2.stop()

The messages don't have to be strings; they can be any Python object.