Unfortunately in this problem, you don't get to use Semaphore. Reader_thread_3 = Thread(target = reader)
Reader_thread_2 = Thread(target = reader) Reader_thread_1 = Thread(target = reader) If the order matters, use a queue, and worker threads: from threading import ThreadĮnqueuer_thread = Thread(target = enqueue) time.sleep(arbitrarilyLargeEnoughNumber) doesn't really work when you have more than 2 concurrent pieces of code, since you don't know which one will run next - see * below. See Controlling scheduling priority of python threads? for more. So my answer would be, you do not want to use semaphores to print (or process) something in a certain order reliably, because you cannot rely on thread prioritization in Python. By default, the interpreter pre-empts a thread every 5ms ( sys.getswitchinterval() returns 0.005), and remember that these threads never run in parallel, because of Python's GIL The answers are using it as a way to get Python's bytecode interpreter to pre-empt the thread after each print line, so that it alternates deterministically between running the 2 threads. Also, these variables are quite arbitrary, which is why each answer has a different value they sleep (block the thread for). If all your threads are sleeping, your app isn't doing anything. This should be avoided in real software, because blocking your thread for 0.25, 0.5 or 1 second is unnecessary/wasteful - you could be doing more processing, especially if your application is IO bound - it already blocks when it does IO AND you are introducing arbitrary delays (latency) in your processing time. I noticed that almost all answers use some form of time.sleep or asyncio.sleep, which blocks the thread. When I run this - I get the following output. This will only work is the synchronization is being done from the same thread, IPC from separate processes will fail using this mechanism. This was written using Python3.6 Not tested on any other version.
#SEMAPHOR PROGRAMMING CODE#
I used this code to demonstrate how 1 thread can use a Semaphore and the other thread will wait (non-blocking) until the Sempahore is available. # Normally, we will wait until all the task has done, but that is impossible in your case.Īwait asyncio.sleep(2) # So, I let the main loop wait for 2 seconds, then close the program. from asyncio import (Īwait asyncio.sleep(0.25) # Transfer control to the loop, and it will assign another job (is idle) to run. In fact, I want to find asyncio.Semaphores, not threading.Semaphore,
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