Need for while True:

Question

I don't understand why "while True:" is needed in below example

  import os
  import sys
  import subprocess
  import time
  from threading import Thread
  from Queue import Queue

  def worker():
     while True:
          item = q.get()
          do_work(item)
          q.task_done()

  def do_work(item):
      time.sleep(item)
      print item


  q = Queue()
  for i in range(2):
       t = Thread(target=worker)
       t.daemon = True
       t.start()

  source = [2,3,1,4,5]

  for item in source:
      q.put(item)

  q.join()

score 12 · Accepted Answer · edited May 23 '17 at 12:07

12

Because otherwise the worker thread would quit as soon as the first job was processed from the queue. The infinite loop ensures that the worker thread retrieves a new job from the queue when finished.

Update: to summarize the comments to my (admittedly hasty) answer: the worker thread is daemonic (ensured by t.daemon = True), which means that it will automatically terminate when there are only daemonic threads left in the Python interpreter (a more detailed explanation is given here). It is also worth mentioning that the get method of the queue on which the worker operates blocks the thread when the queue is empty to let other threads run while the worker is waiting for more jobs to appear in the queue.

edited May 23 '17 at 12:07

Community

1
1

answered Jun 20 '11 at 19:08

Tamás

47,239
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1

+1 Also worth adding that presumably `q.get()` blocks when the queue is empty thus avoiding running a busy loop – David Heffernan Jun 20 '11 at 19:09
2

I think it worths to mention, that it doesn't fall into an infinite loop, because the worker thread is daemonic. – khachik Jun 20 '11 at 19:12
Thanks guys, I've extended my answer to include this. – Tamás Jun 20 '11 at 19:18

Need for while True:

1 Answers1