In Python 3, how can I run two functions at the same time?

Question

I have 2 functions: one that reads a text file, and the other that changes that text file when a key is pressed. I need my code to print the text file every few seconds, and at the same time, monitor for key presses to change the file. Is this possible and how could I do it?

I've tried this, but it doesn't work.

def read_file():
   color = open("color.txt", "r")
   current_color = color.read()
   color.close()
   if current_color == "green":
       print("GREEN")
   elif current_color == "blue":
       print("BLUE")
   time.sleep(5)

def listen_change():
   if keyboard.is_pressed('g'):
       f = open("color.txt", "w")
       f.write("green")
       f.close()
   elif keyboard.is_pressed('b'):
       f = open("color.txt", "w")
       f.write("blue")
       f.close()

EDIT: here's how I tried multithreading

from threading import Thread

if __name__ == '__main__':
    Thread(target=read_file()).start()
    Thread(target=listen_change()).start()

The target argument should be a function, not a call to the function. Remove the `()` — Barmar, Jan 12 '21 at 05:10
Trying it without the `()` throws `RuntimeError: cannot schedule new futures after interpreter shutdown`. This might be outside my knowledge, not sure what a mutex is, — aidenmitchell, Jan 12 '21 at 05:20
mutex = mutual exclusion, it's used to avoid both functions trying to acess the file at the same time. Otherwise, the reading function might read the file while the writing function has only partially written it. — Barmar, Jan 12 '21 at 05:21
See https://stackoverflow.com/questions/51879070/python-executor-spawn-tasks-from-done-callback-recursively-submit-tasks — Barmar, Jan 12 '21 at 05:26

score 0 · Accepted Answer · answered Jan 12 '21 at 05:43

The target argument has to be a function. You're calling the functions immediately, not passing references to the function.

The functions need to loop.

Use join() to wait for the threads to exit.

Use a lock to prevent reading and writing the file at the same time, which might read the file in a partial state.

import keyboard
from threading import Thread, Lock

mutex = Lock()

def read_file():
   while True:
       with mutex:
           with open("color.txt", "r") as color:
               current_color = color.read()
       if current_color == "green":
           print("GREEN")
       elif current_color == "blue":
           print("BLUE")
       time.sleep(5)

def listen_change():
   while True:
       if keyboard.is_pressed('g'):
           with mutex:
               with open("color.txt", "w"):
                   f.write("green")
       elif keyboard.is_pressed('b'):
           with mutex:
               with open("color.txt", "w"):
                   f.write("blue")

if __name__ == '__main__':
    t1 = Thread(target = read_file)
    t2 = Thread(target = listen_change)
    t1.start()
    t2.start()
    t1.join() # Don't exit while threads are running
    t2.join()

In Python 3, how can I run two functions at the same time?

1 Answers1