GUI and multithreading in Python: pause and restart the loop in the stream

Question

It became necessary to deal with multithreading in the GUI. Please look at my version and tell how far it is from the correct implementation, suggest about the general principles of managing the flow of GUI elements. The program is used to work with the terminal.

ser = serial.Serial('COM4') bills = [0, 0, 10, 50, 100, 500, 1000, 5000, 1, 2, 5, 10] ev = threading.Event() q = multiprocessing.Queue() def polling_loop(): while True: # у меня есть часть кода, которая должна выполнятся один раз при каждом 'включении' потока ev.wait()# приостанавливаю поток # инициализирую купюроприемник ser.write(b'\x02\x03\x06\x30\x41\xb3') # здесь код, который выполняется один раз time.sleep(0.2) ser.write(b'\x02\x03\x0c\x34\xff\xff\xff\x00\x00\x00\xb5\xc1') while ev.is_set(): # а здесь код, который выполняется, # пока я не сброшу флаг с event`а кнопкой # здесь в цикле идет опрос на получение банкнот ser.write(b'\x02\x03\x06\x00\xc2\x82') ser.write(b'\x02\x03\x06\x33\xda\x81') time.sleep(0.2) if ser.inWaiting() > 0: resp = ser.read(ser.inWaiting()) if resp[3] == 129: q.put(bills[resp[4]]) def stop(): ev.clear() def start(): ev.set() class Window(Tk): def __init__(self, *args, **keywords): super().__init__(self, *args, **keywords) self.geometry('20x200') self.start = Button(text="start", command=lambda: start()) self.stop = Button(text="stop", command=lambda: stop()) self.stop.pack() self.start.pack() th2.start() # поток этот запускается сразу при инициализации th2 = threading.Thread(target=polling_loop) tk = Window() tk.mainloop() th2.join()

describe in words what you expect the code to do and what happens instead.
If your code works correctly, and you just want a response on a specific implementation of a code for a specific task, still clearly describe what you think it does and add a code inspection code (be sure that the external (observed) code behavior is correct (with your point of view), in case you use this tag)
(I also recommend sticking a fully working code into the question so that I don’t have to think it out myself)
The code works as I need, but I’m not sure that my version is correct, as I didn’t have anything to do with the threads before, but I don’t really like writing according to the principle “it works and it’s okay”
Related Questions: Is there a way to kill a thread in python?

Accepted Answer · 2016-08-31T12:08:45

 import threading, tkinter, time class Window(tkinter.Tk): def __init__(self): tkinter.Tk.__init__(self) self.geometry('20x200') self.is_run = False tkinter.Button(text="start", command=lambda: self.is_run or threading.Thread(target=self.run).start()).pack() tkinter.Button(text="stop", command=self.stop).pack() def stop(self): self.is_run = False def run(self): self.is_run = True print('у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока') while self.is_run: print('а здесь код, который выполняется') time.sleep(1) print('выход') if __name__ == '__main__': Window().mainloop()

out:

 у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока а здесь код, который выполняется а здесь код, который выполняется а здесь код, который выполняется выход

or

 import threading, tkinter, time, queue class SThread(threading.Thread): '''worker поток''' def __init__(self): threading.Thread.__init__(self) self.queue = queue.Queue() self.setDaemon(True) self.start() def run(self): print('thread.run') while True: cmd = self.queue.get(timeout=10) print(cmd) if cmd is None: break cmd['out'] = cmd['target'](*cmd.get('args', ()), **cmd.get('kwargs', {})) self.queue.task_done() class Window(tkinter.Tk): def __init__(self): tkinter.Tk.__init__(self) self.geometry('20x200') self.is_run = False self.thread = SThread() tkinter.Button(text="start", command=lambda: self.is_run or self.thread.queue.put( dict(target=self.run))).pack() tkinter.Button(text="stop", command=self.stop).pack() def stop(self): self.is_run = False def run(self): print('у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока') self.is_run = True while self.is_run: print('а здесь код, который выполняется') time.sleep(1) print('выход') if __name__ == '__main__': Window().mainloop()

out:

 thread.run {'target': <bound method Window.run of <__main__.Window object at 0x01F19710>>} у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока а здесь код, который выполняется а здесь код, который выполняется выход

or

 import threading, tkinter, time class Window(tkinter.Tk, threading.Thread): def __init__(self): tkinter.Tk.__init__(self) threading.Thread.__init__(self) self.wait = 1 self.setDaemon(True) self.start() self.geometry('20x200') tkinter.Button(text="start", command=self.Start).pack() tkinter.Button(text="stop", command=self.Stop).pack() def destroy(): self.wait = -1 time.sleep(1) self.destroy() self.protocol("WM_DELETE_WINDOW", destroy) def Stop(self): self.wait = 1 def Start(self): self.wait = 0 def run(self): print('thread.start') while self.wait >= 0: if self.wait: time.sleep(self.wait) else: self._target = self.work self._args = () self._kwargs = {} super().run() print('thread.stop') def work(self): print('у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока') while not self.wait: print('а здесь код, который выполняется') time.sleep(1) print('выход') if __name__ == '__main__': Window().mainloop()

out:

 thread.start у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока а здесь код, который выполняется а здесь код, который выполняется выход у меня есть часть кода, которая должна выполнятся один раз при каждом включении потока а здесь код, который выполняется выход thread.stop

Super (). Run () method

 def run(self): """Method representing the thread's activity. You may override this method in a subclass. The standard run() method invokes the callable object passed to the object's constructor as the target argument, if any, with sequential and keyword arguments taken from the args and kwargs arguments, respectively. """ try: if self._target: self._target(*self._args, **self._kwargs) finally: # Avoid a refcycle if the thread is running a function with # an argument that has a member that points to the thread. del self._target, self._args, self._kwargs

This option was considered by me, but it does not fit, because the thread can not be started a second time.
why not, run it like this: threading.Thread (target = ThrObj.run) .start ().

Community spirit ♦ one · Answer 2 · 2016-09-09T11:59:44

Using a background thread to execute a blocking code in a GUI program and threading.Event to pause / launch code from another thread is fully justified. Although, if the Event.wait(timeout) method is not needed in your case, then even a simple True / False flag could be used instead of threading.Event() .

Sometimes a separate thread is not needed at all: work with I / O can be built into the cycle of GUI events. Tkinter provides the Tk.createfilehandler() method on some systems. Sample code that createfilehandler() uses to read output from a subroutine . For comparison, sample code that uses the background thread for the same task — pay attention as the Tk.after() method is called to create a loop in the GUI thread (without blocking the thread itself).

A full example that pauses and restarts the loop in a separate thread:

 #!/usr/bin/env python3 import queue import threading def restartable_loop(restarted, results): while restarted.wait(): # wait until the loop is restarted results.put("часть кода, которая должна выполнятся один раз " "при каждом 'включении' потока") while restarted.is_set(): # while it is running results.put('код, который выполняется, ' 'пока флаг restarted не переустановлен') results.put(None) # mark the end of the current batch q = queue.Queue(maxsize=10) restarted = threading.Event() run_loop = restarted.set pause_loop = restarted.clear print('запускаем поток') threading.Thread(target=restartable_loop, args=[restarted, q], daemon=True).start() try: q.get(timeout=.5) except queue.Empty: # ничего не должны получить пока цикл не запущен pass else: assert 0, "never happens" for _ in range(2): # повторим пару раз для демонстрации print('запускаем цикл') run_loop() print(q.get()) # получаем результат print(q.get()) # получаем результат print('приостанавливаем цикл') pause_loop() print('считаем сколько результатов получили:', len(list(iter(q.get, None))))

Sample output

 we start a flow
 start the cycle
 a piece of code that should be executed once each time the stream is 'turned on'
 code that runs until the restarted flag is reset
 suspend cycle
 consider how many results were received: 11
 start the cycle
 a piece of code that should be executed once each time the stream is 'turned on'
 code that runs until the restarted flag is reset
 suspend cycle
 we consider how many results received: 9

Remarks on the code:

use queue.Queue instead of multiprocessing.Queue (threads and processes are different things)
Do not use time.sleep() in code with many threads if you can find more explicit synchronization mechanisms between threads. For example, in the sample code, the response uses the blocking Queue.put() method . Thanks to Queue(maxsize=10) , the code in the background thread will pause if the main thread does not have time to remove the elements from the queue in time.
use command=start , instead of command=lambda: start() . lambda not needed in this case — in Python, functions can be passed as-is as parameters to other functions
in general, prefer delegation instead of inheritance. For example, tkinter.Tk() passed as a parameter to ShowProcessOutputDemo in the example code above, instead of using Tk as the base class: inheritance creates too close a link between the base and child classes (for example, you have to think about conflicts of non-private class attributes) —logics the application and logic of a reusable GUI element (window) may make sense to keep it separate.

Thank you for your participation. This is all still relevant for me, so thank you very much

GUI and multithreading in Python: pause and restart the loop in the stream

2 answers 2

Sample output

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