A quick demo based on A. Jesse Jiryu Davis' PyCon 2017 "Grok the Gil" talk (https://www.youtube.com/watch?v=7SSYhuk5hmc)

grok.py

import threading

n = 0


def foo():
    global n
    n += 1


def test(iterations):
    global n
    n = 0
    threads = []
    for i in range(iterations):
        t = threading.Thread(target=foo)
        threads.append(t)

    for t in threads:
        t.start()

    for t in threads:
        t.join()

    print(n)


def main():
    for iterations in (10**x for x in range(2, 6)):
        raw_input("Let's try with {}".format(iterations))
        for _ in range(10):
            test(iterations)


if __name__ == '__main__':
    main()