Python: priority queue with time as priority

Question

I have a priority queue build using heaps. The queue contains messages, which should be send in the order regarding to the priority. However, as the priority value I have a time after which the message should be send, for example a set of messages which I have to put to the queue:

(10, message1)
(15, message2)
(5, message3)

So sending the messages following the priority is easy. However, if I will first send the messag3 after 5 seconds from putting it to the queue, I would like to make sure that the next message, message1, will be send 10 seconds after putting into the queue, what gives 5 seconds after message3 was send. Does anyone knows any examples how I can manage to do that?

Answer 1

You could use epoch as a priority value and every time timer fires calculate when it should fire again based on current time. Here's a short example of that in practice:

import calendar
import time
import heapq
from threading import Timer

def epoch():
    return calendar.timegm(time.gmtime())

start_time = epoch()
heap = []
timer = None

def add_message(seconds, content):
    top = heap[0] if heap else None
    heapq.heappush(heap, (epoch() + seconds, content))
    if timer and top != heap[0]:
        timer.cancel()
        start()

def start():
    global timer
    if heap:
        timer = Timer(heap[0][0] - epoch(), fire)
        timer.start()

def fire():
    _, message = heapq.heappop(heap)
    print '{}: {}'.format(epoch() - start_time, message)
    start()

add_message(10, 'message1')
add_message(15, 'message2')
add_message(5, 'message3')
start()
add_message(1, 'message4')

Output:

1: message4
5: message3
10: message1
15: message2