CODEWITHSUNDEEP
pythonmultithreading
quant
quant

Reputation: 2214

Python multithreading.Process Lock

I'm trying to understand the Lock Object in Python. For this, I have programmed, something like the python multithreading.Pool object. However I'm still a bit wondering why the lock don't really seem to lock the execution of the other Processes. If I replace class DoSomething(multiprocessing.Process): by class DoSomething(threading.Thread): this works perfectly (yeah, ok, only if there's no exception, because pop don't work on an empty list); but I'm not sure, wheather this is because of the lock, or because of the shared memory.

#! /usr/bin/env python

import multiprocessing, threading

lock = multiprocessing.Lock()

class DoSomething(multiprocessing.Process):

    def __init__(self, jobList, lock):
        super(DoSomething, self).__init__()
        self.jobList, self.lock = jobList, lock

    def run(self):

        while jobList != []:               # while there're still jobs available
            self.lock.acquire()            # lock the jobList
            job = self.jobList.pop()       # and take the next job (in fact, pop removes the
            print job                      # lst object from the list and returns it).
            self.lock.release()            # release the jobList, so that another thread may do another job
            # do something with this job ...

threads = []
jobList = [1,2,3,4]
for i in range(4):
    threads.append(DoSomething(jobList, lock))
    threads[-1].start()

for t in threads:
    t.join()

The output of this program is:

$ python multThread.py 
4
3
2
1
4          // so it don't lock, we have calculated the job twice here!
3
2
1
// ...

Upvotes: 0

Views: 208

Answers (1)

dhui
dhui

Reputation: 522

The lock is working. The problem is that, with multiprocessing, the jobList is not in shared memory like it is with threading. In multiprocessing, the separate processes have a copy of the jobList. Try making the jobList a multiprocessing.Array. See Sharing state between processes for more info.

Upvotes: 1

Related Questions