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from __future__ import with_statementfrom heapq import heappush, heappopfrom time import timefrom threading import Event, Lockimport Queueclass Sched:    def __init__(self):        self.heap = []        self.running = True        self.update_event = Event()        self.heap_lock = Lock()    def add(self, delay, queue):        """Add new execution queue"""        with self.heap_lock:            heappush(self.heap, (time() + delay, delay, queue))        self.update_event.set()    def stop(self):        self.running = False        self.update_event.set()    def tasks(self):        """Produce events until stopped"""        self.running = True        while self.running:            self.update_event.clear()            if not len(self.heap):                self.update_event.wait()            with self.heap_lock:                runtime, delay, queue = heappop(self.heap)            if time() < runtime:                with self.heap_lock:                    heappush(self.heap, (runtime, delay, queue))                self.update_event.wait(runtime - time())            else :                try :                    task = queue.get_nowait()                except Queue.Empty:                    pass                else :                    yield task                    queue.task_done()                    with self.heap_lock:                        heappush(self.heap, (time() + delay, delay, queue))    def raced_tasks(self):        """Produce events until stopped"""        self.running = True        while self.running:            self.update_event.clear()            if not len(self.heap):                self.update_event.wait()            runtime, delay, queue = checked_event = self.heap[0]            now = time()            if now < runtime:                self.update_event.wait(runtime - now)            else :                runtime, delay, queue = event = heappop(self.heap)                # assure event was not removed or altered after last look at self.heap[0]                if event is checked_event:                    try :                        task = queue.get_nowait()                    except Queue.Empty:                        continue                    else :                        yield task                        queue.task_done()                self.add(delay, queue)if __name__ == '__main__':    s = Sched()    half_sec_q = Queue.Queue()    half_sec_q.put(lambda :'foo')    half_sec_q.put(lambda :'bar')    s.add(0.5, half_sec_q)    two_sec_q = Queue.Queue()    two_sec_q.put(lambda :'baz')    s.add(2, two_sec_q)    three_sec_q = Queue.Queue()    three_sec_q.put(s.stop)    s.add(3, three_sec_q)    print time()    for value in s.tasks():        print time(), value()