Python - Threading

[last updated: 2022-10-21]
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• Run two python programs at the same time:
  • 3 options:
    • Threading module
    • Multiprocessing module
    • running programs concurrently with script files

  • These three methods all do sort of the same thing, in a broad categorical sort of way,
    but of course they're different, with different features, requirements, and tradeoffs.

  • Python has a GIL = Global Interpreter Lock
    that is intended to prevent running two threads at the same time.
    It must be defeated or tricked in order to run such programs.

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• Threading:
  • I'll start with this one, because this is the one that has the features I need for my current project:
    Run two program loops at the same time, and pass parameters/data between them:
  • threading is the high-level module, and
    _thread is the lower level module.

  • Threads may or may not be in daemon mode.
    When you create a thread, it starts as a non-daemon thread by default.
    You can change a thread to daemon mode with:
    d = threading.Thread(target=dFunc)
    d.setDaemon(True)

  • Threads may or may not be joined.
    By default they are not joined when they are created, but you can join them with;
    d.join()

  • Two possible daemon states x two possible join states results in 4 possibilities:
    • Non-daemon, not joined:
      If the created thread has not finished its program steps when the main thread is ready to exit,
      then the main thread will exit, leaving the created thread still running to its natural conclusion/exit.
    • Daemon thread, not joined:
      The daemon thread will be killed when the main program quits,
      even if it hasn't finished its programmed steps.
    • Non-daemon, but joined:
      The main program will block/wait (at the place in the code where the join command is)
      until the created/joined thread is done, then it will resume and proceed past the join command to finish its code.
    • Daemon, and joined:
      Same as non-daemon and joined.

  • You can add a timeout argument to the join command, which specifies how long the main thread will wait for the target (joined) thread to finish.
    • If the main thread has blocked/paused at the join command,
      it will wait there without executing any more statements, but for no longer than the timeout period.
    • If target thread finishes before the end of the timeout period, the main thread will immediately exit.
    • If the target thread does not exit before the end of the timeout period,
      then the main thread will regardless exit,
      leaving the target thread still active and running (just as if it were not joined at all).
      Unless as stated above in the case that the created thread is in daemon mode,
      in which case it will immediately terminate when the main thread does.

  • Locks and Semaphores:
    • ...

    ---------------------------------------------------------

  • from some forum...:
    "If not following a command queue/message pump model (using the Queue module), then manual use of synchronization primitives become a necessity (decisions are needed for the granularity of locking)"

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• Links:
  • google: python why use classes

  • docs.python.org - full documentation for threading module... https://docs.python.org/3.10/library/threading.html
  • tutorial - a little deep, only partly useful... https://www.toptal.com/python/beginners-guide-to-concurrency-and-paralle...
  • stackexchange - user problem running threading, solution is to use multiprocessing instead...
        https://raspberrypi.stackexchange.com/questions/75687/raspberry-pi-pytho...
  • rPi forum - quick & basic, only partly helpful... https://forums.raspberrypi.com/viewtopic.php?t=328771
  • stackoverflow - join command... https://stackoverflow.com/questions/15085348/what-is-the-use-of-join-in-...
  • stackoverflow - good, relevant, pretty thick... https://stackoverflow.com/questions/64199897/python-threads-on-raspberry...
  • makergram - good, basic, relevant example... https://makergram.com/community/topic/29/multi-thread-handling-for-norma...
  • packt - very good, esp examples of lock and join but leaves hanging and must buy subscription for more detail
        https://subscription.packtpub.com/book/hardware-and-creative/97817835515...
  • robsBlog - very good basic example... http://robsraspberrypi.blogspot.com/2016/01/raspberry-pi-python-threadin...
  • realPython - good but need to register... https://realpython.com/intro-to-python-threading/
  • bogoToBogo - excellent... https://www.bogotobogo.com/python/Multithread/python_multithreading_Daem...
  • stackoverflow - threading vs. multi-processing... https://stackoverflow.com/questions/3044580/multiprocessing-vs-threading...
  • docs.python.org - full documentation for multiprocessing module... https://docs.python.org/3.10/library/multiprocessing.html
  • stackoverflow - running concurrent bash scripts... https://stackoverflow.com/questions/28549641/run-multiple-python-scripts...
  • using _thread... https://www.electrosoftcloud.com/en/multithreaded-script-on-raspberry-pi...
  • https://superfastpython.com/join-a-thread-in-python/
  • https://superfastpython.com/threading-in-python/
  • https://stackoverflow.com/questions/23100704/running-infinite-loops-usin...

    https://pypi.org/project/continuous-threading/
    https://betterprogramming.pub/how-to-time-threaded-python-loops-efficien...
    https://rednafi.github.io/reflections/use-daemon-threads-to-test-infinit...
    https://www.quora.com/Is-it-better-to-put-a-while-true-loop-inside-a-thr...
    https://gist.github.com/bennuttall/6bedcb118f8c031920cba31936f79441
    https://lawsie.github.io/guizero/blocking/
    https://stackoverflow.com/questions/59018383/guizero-raspberry-pi-button...
    https://www.oreilly.com/library/view/python-cookbook/0596001673/ch09s07....
    https://github.com/lawsie/guizero/tree/master/examples
    https://superfastpython.com/lock-vs-semaphore-in-python/
    https://gist.github.com/bennuttall/6bedcb118f8c031920cba31936f79441

  • Good links but not relevant to threading:
    • ucsd - binding a variable to a type - good...
          https://cseweb.ucsd.edu/classes/sp05/cse130/lecture_notes/types_student.txt
    • tutorialsPoint - binding in tkinter - good...
          https://www.tutorialspoint.com/tkinter-binding-a-function-with-arguments...
    • python-course.eu - binding - good...
          https://python-course.eu/tkinter/events-and-binds-in-tkinter.php
    • python-forum - hover in tkinter... https://python-forum.io/thread-25802.html

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