How to pass a boolean by reference across threads and modules

Question

I have a boolean that I want to pass to different threads that are executing methods from different modules. This boolean acts as a cancellation token so if set, the thread should exit. It seems to be passed by value since if I set it in another thread it doesn't change in the other threads. Thanks.

import module2
from threading import Thread

cancellationToken = False

def main:
    thread2 = Thread(target = module2.method2, args (on_input, cancellationToken, ))
    thread2.start()
    ...
    thread2.join()

def on_input(command):
    global cancellationToken
    ...
    if(...):
        cancellationToken = True
    ...

method2 in module2 is just a simple infinite while loop that checks the cancellation token and responds to user input.

def method2(on_input, cancellationToken):
    while(True):
        if(cancellationToken):
            return
        ...
        on_input(...)

Answer 1

When you do this:

thread2 = Thread(target = module2.method2, args (on_input, cancellationToken, ))

You're essentially passing the value False for the 2nd argument to the thread method.

But when you do this after that:

cancellationToken = True

You're replacing the reference represented by cancellationToken, but not the value that was originally passed to thread2.

To achieve what you want to do, you'll need to create a mutable object wrapper for your cancellation state:

class CancellationToken:
   def __init__(self):
       self.is_cancelled = False

   def cancel(self):
       self.is_cancelled = True

cancellationToken = CancellationToken()

thread2 = Thread(target = module2.method2, args (on_input, cancellationToken, ))

# then later on
cancellationToken.cancel()

Your thread code becomes:

def method2(on_input, cancellationToken):
    while(True):
        if(cancellationToken.is_cancelled):
            return
        ...
        on_input(...)