Sometimes it doesn't appear at all, and sometimes it'll be printed twice. When you run the above, most of the time the 'queue is still empty' appears once. If you add a loop to check for emptyness first then your code works: queue = multiprocessing.Queue() After putting an object on an empty queue there may be an infinitesimal delay before the queue’s empty() method returns False.This has some consequences which are a little surprising, but should not cause any practical difficulties – if they really bother you then you can instead use a queue created with a manager. When an object is put on a queue, the object is pickled and a background thread later flushes the pickled data to an underlying pipe. See the note in the Pipes and Queues section: The implementation is a bit more involved to support communication between multiple processes, so threads and pipes are involved that cause the empty state to last a little longer than your code allows for. That's because the queue is not instantly not empty. Your code actually works, some of the time. The same applies if multiple processes are reading from the queue.įor single-producer / single-consumer scenarios, using a multiprocessing.Pipe instead of multiprocessing.Queue would be sufficient and more performant, though. Since a thread could drop the GIL in between checking if not queue.empty() and queue.get(), this wouldn't be suitable for multi-threaded queue-reads in a process. If not queue.empty(): # this is not an atomic operation. In case only one process and only one thread within this process is reading the queue, it would be also possible to exchange the last code snippet with: while True: or use get_nowait() and handle a possible queue.Empty exception if you need a non-blocking solution. iter(callable, sentinel)? from multiprocessing import Queue get() from the queue, where you break out of the loop by passing a sentinel value. docsĮither use for msg in iter(queue.get, sentinel): to. Because of multithreading/multiprocessing semantics, this is not reliable. The multiprocessing.Queues module offers a Queue implementation to be used as a message passing mechanism between multiple related processes. Return True if the queue is empty, False otherwise. So here is the answer reloaded, which is able to deal with the usual interleaved feeds & reads scenarios:ĭon't rely on queue.empty checks for synchronization.Īfter putting an object on an empty queue there may be an infinitesimal delay before the queue’s empty() method returns False and get_nowait() can return without raising queue.Empty. The extended demonstration code Martijn showed, wouldn't work in the usual scenarios, because the while loop would break too soon when enqueuing doesn't keep up with reading. No child process involved for demonstration, but because in real applications hardly ever a queue is pre-filled and only read out after, but readingĪnd writing happens interleaved with waiting times in between. I realized, that the use case in OP's example doesn't quite fit to the canonical sounding title of I originally deleted this answer after I read Pieters', since he decribed the "why this doesn't work" in more detail and earlier. Process ( target = socket2, args = ( flag, key ,)) p1. Process ( target = socket1, args = ( flag, key, size )) p2 = mp. Import multiprocessing as mp import time def pipe1 ( * args ): p = args key = args size = args d = print ( " = 5 break def main_socket ( key, size = 10000 * 10000 ): flag = mp.
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