How to listen event with Python threading

Question

I’m trying to implement a multi thread program in python and am having troubles.
I try to design a program, when the program (main thread) receives a specific command,
The counter of the program will return to the previous number and continue counting down.

The following is the code I tried to write:

import threading
import time
count = 0
preEvent = threading.Event()
runEvent = threading.Event()
runEvent.set()
preEvent.clear()


def pre():
    global count
    while True:
        if event.isSet():
            count -= 1
            event.clear()


def doSomething():
    global count
    while True:
        # if not runEvent.isSet():
        #   runEvent.wait()
        print(count)
        count += 1
        time.sleep(1)


def main():
    t1 = threading.Thread(target=pre, args=())
    t2 = threading.Thread(target=doSomething, args=())
    t1.start()
    t2.start()
    command = input("input command")
    while command != 'quit':
        command = input("input command")
        if command == 'pre':
            preEvent.set()

main()

But I encountered a few problems

How to block t1 simultaneously while I inputting a specific command
How to start from the beginning when t1 is restored instead of starting from the blocked point

Regarding question 1, I tried adding a condition check before the print(count) command, but if I enter the "pre" command when the program outputs count, the program will still perform count+=1
After that, the program will go back to check the conditions and block t1, but this does not achieve the synchronization effect I wanted.
Is there a way to achieve this goal?

Question 2 is similar to question 1. I hope that whenever I enter the command, my program will output like this.

1
2
3
4
pre
3
4
...

But if t1 is blocked after finishing the print(count) instruction, when the event is cleared, t1 will continue to execute from count+=1
So the output will become the following

1
2
3
4
pre
4
5
...

I tried to find information on the Internet, but I never knew how to add keywords.
Is there a method or library that can achieve this function?

I have tried my best to describe my problem, but it may not be good enough. If I have any questions about my problem, I can add more explanation.

Thank you all for your patience to read my question

Asked By: foogty

||

Source

Answer 1

The threading module has a wait method that will block until notify or notify_all is called. This should accomplish what you’re looking for in your first question. For question 2 you can either define a function that handles the exit case, or just recreate a thread to start form the beginning.

Answered By: Nate

Answer 2

Simply it can work like this, maybe helps.

from threading import Thread, Event

count = 0
counter = Event()

def pre():

    global count
    counter.wait()
    count -= 1
    print(count)

def main():

    global count
    print(count)
    while True:
        command = input("Enter 1 to increase, Enter 2 to decrease :")
        if command == "1":
            t1 = Thread(target=pre, args=())
            t1.start()
            counter.set()
        else :
            count += 1
            print(count)


main()

Answered By: brfkorucu

How to listen event with Python threading

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