How can I modify variables across different threads in Python?

Question

I can't seem to change the x or y values in the code below. I thought the second thread will wait until the the calculation is complete?

I don't know what fundamentally I'm doing wrong here?

from threading import Event, Thread
import numpy as np

def test():
    x = [0, 1]
    y = [1, 3]

    def calc_callback(ev):
        x = np.linspace(-5, 5, 100)
        y = np.sin(x)/x
        ev.set()


    def display_callback(ev):
        ev.wait()
        print(x)
        print(y)

    completion_event = Event()
    Thread(target=calc_callback, args=[completion_event]).start()
    Thread(target=display_callback, args=[completion_event]).start()


if __name__ == '__main__':
    test()

Michael Butscher · Accepted Answer · 2019-02-01 23:43:24Z

2

Using x = assignment in calc_callback creates a new variable x independent of the x in enclosing test(). Only this new variable is modified and then thrown away (same for y).

Try nonlocal declaration (needs Python 3.x):

[...]

    def calc_callback(ev):
        nonlocal x, y
        x = np.linspace(-5, 5, 100)
        [...]

answered Feb 1, 2019 at 23:43

Michael Butscher

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1 Comment

Ethan Over a year ago

It works! Thanks. I'll have to read up more on nonlocal declaration.

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