Question

I tried to put together a very simple multithreading model, and so far it seems to work. My question is how am I sure that two threads will not grab the same value from the queue simultaneously and give me repeats? Is there just some built in method that prevents this? I added a delay to try to put time between when each thread will get a value from the queue, is this necessary?

from Queue import Queue
from threading import Thread
import time

class myThread(Thread):
    def __init__(self,queue):
        Thread.__init__(self)
        self.queue = queue
    def run(self):       
        while True:                      
            time.sleep(0.0001)   #not sure if this is good enough to compensate for overlap in pulling the same value out of a queue         
            task = self.queue.get() #pull a number from the queue,
            if task != None:          
                out.append(task) #This will be where you        
            else:               
                break


queue = Queue()
out = []


for i in range(10):
    t = myThread(queue)
    t.start()
for i in range(100):
    queue.put(i)
print out

Answer 1

The Queue class implements locking to prevent this happening, see http://docs.python.org/library/queue.html, in particular:

It is especially useful in threaded programming when information must
be exchanged safely between multiple threads. The Queue class in this
module implements all the required locking semantics.

So you don’t need any delay, the queue should work exactly as you want. That’s what it was written for (basically) 🙂