Example of a data race in Python

python_data_race.py

# Does Python have data races?
#
# As we've seen, Java can have data races, according to
# https://docs.oracle.com/javase/specs/jls/se8/html/jls-17.html
# It says an example of a data race is when:
# - there is a write in one thread,
# - a read of the same variable by another thread,
# - and the write and read are not ordered by synchronization.
#
# This program attempts to cause a data race in Python.

from threading import Thread
from time import sleep

counter = 0

def increase():
    global counter
    for i in range(0, 100000):
        counter = counter + 1

threads = []
for i in range(0, 400):
    threads.append(Thread(target=increase))
for thread in threads:
    thread.start()
for thread in threads:
    thread.join()

print(f'Final counter: {counter}')

# A few trials from my computer (Mac, 2.6 GHz 6-Core i7):
# Final counter: 31735072
# Final counter: 32829326
# Final counter: 31496003
#
# We conclude that yes, Python has data races.

If you're (like me) struggling to recreate this, here's an easy change to show the preemptive multitasking creating race conditions (works every time for me):

def increase():
    global counter
    for i in range(0, 10):
        val = counter
        sleep(0.0001)
        counter = val + 1

You can also play around with sys.setswitchinterval() that changes the interval for thread switching