How to multithread using threading

Question

The queueLock.acquire() line hangs a code that tries to take advantage of threading and Queue modules with following output:

run() on <newThread(Thread-1, started 4344102912)>

run() on <newThread(Thread-2, started 4348309504)>

...working delay: 1

Why?

import threading, thread, Queue, time
class newThread (threading.Thread):
 def __init__(self, delay=0, workQueue=None):
 self.delay=delay
 self.workQueue=workQueue
 threading.Thread.__init__(self)
 def run(self):
 print '\nrun() on %s'%self
 print_time(self.delay, self.workQueue)
 print '\nrun() exit %s'%self
def print_time(delay=None, workQueue=None):
 def onExit():
 if not workQueue.empty():
 data = workQueue.get()
 queueLock.release()
 else:
 queueLock.release()
 counter=0
 while True:
 queueLock.acquire()
 time.sleep(delay)
 print '\t...working delay: %s'%delay 
 if counter>5:
 onExit()
 counter=counter + 1
queueLock = threading.Lock()
workQueue = Queue.Queue(10)
threads = []
thread1 = newThread(delay=1, workQueue=workQueue)
thread1.start()
threads.append(thread1)
thread2 = newThread(delay=2, workQueue=workQueue)
thread2.start()
threads.append(thread2)
queueLock.acquire()
print '1. workQueue.empty():',workQueue.empty()
workQueue.put('One')
print '2. workQueue.empty():',workQueue.empty()
workQueue.put('Two')
queueLock.release()
#Wait for queue to empty
while not workQueue.empty():
 print '...passing while not workQueue.empty()'
 pass
for thread in threads:
 thread.join()
print '...completed'

• Oliver Dain: Error message and stack trace would be very helpful. It would also help to show what the output is (i.e. which of your print lines actually executed)
• T.D. Smith: Are you running standard C-based Python? If so, you can't do multithreading in the traditional sense. Look at the GIL
• jfs: @Tyler: GIL has nothing to do with it.
• Ulrich Eckhardt: Some suggestions for your code: You don't have to derive from Thread, you can simply run any function with a plain Thread instance. It would also help if you didn't specify default parameters for functions where they may no sense at all, like passing None as queue to print_time(). Also, upgrade to Python 3!
• alphanumeric: Please post some example on how to run any function with a plain Thread instance.

Answer 1

queueLock.acquire() blocks until queueLock.release() is called if queueLock is already acquired. counter > 5 never happens because even if queueLock is available on the first iteration of the loop; nothing releases it on the 2nd iteration.

Queue() has its own locks. Drop queueLock completely.

• Ulrich Eckhardt: I'd like to add that in order to not forget releasing a lock, you can use it as context manager, i.e. with some_lock.acquire(): ....