How do I limit the number of active threads in python?

Question

Am new to python and making some headway with threading - am doing some music file conversion and want to be able to utilize the multiple cores on my machine (one active conversion thread per core).

class EncodeThread(threading.Thread):
    # this is hacked together a bit, but should give you an idea
    def run(self):
        decode = subprocess.Popen(["flac","--decode","--stdout",self.src],
                            stdout=subprocess.PIPE)
        encode = subprocess.Popen(["lame","--quiet","-",self.dest],
                                stdin=decode.stdout)
        encode.communicate()

# some other code puts these threads with various src/dest pairs in a list

for proc in threads: # `threads` is my list of `threading.Thread` objects
    proc.start()

Everything works, all the files get encoded, bravo! ... however, all the processes spawn immediately, yet I only want to run two at a time (one for each core). As soon as one is finished, I want it to move on to the next on the list until it is finished, then continue with the program.

How do I do this?

(I've looked at the thread pool and queue functions but I can't find a simple answer.)

Edit: maybe I should add that each of my threads is using subprocess.Popen to run a separate command line decoder (flac) piped to stdout which is fed into a command line encoder (lame/mp3).

Answer 1

If you want to limit the number of parallel threads, use a semaphore:

threadLimiter = threading.BoundedSemaphore(maximumNumberOfThreads)

class EncodeThread(threading.Thread):

    def run(self):
        threadLimiter.acquire()
        try:
            <your code here>
        finally:
            threadLimiter.release()

Start all threads at once. All but maximumNumberOfThreads will wait in threadLimiter.acquire() and a waiting thread will only continue once another thread goes through threadLimiter.release().