"""
The purpose of this file is to demonstrate how one might use a lock to obtain mutual exclusion.
"""

from threading import Thread, Lock
from time import sleep

def slowChange(thing):
  for i in range(len(thing)):
    value = thing[i]
    sleep(0.01) # simulate a lot of work here
    thing[i] = value + i

wrong = [0,0,0,0]
ts = [Thread(target=slowChange, args=(wrong,)) for _ in range(10)]
for t in ts: t.start()
for t in ts: t.join()
print('wrong:',wrong)


def lockedSlowChange(thing, lock):
  for i in range(len(thing)):
    with lock: # this block can only be run by one thread at a time
      value = thing[i]
      sleep(0.01) # simulate a lot of work here
      thing[i] = value + i

right = [0,0,0,0]
lock = Lock()
ts = [Thread(target=lockedSlowChange, args=(right,lock)) for _ in range(10)]
for t in ts: t.start()
for t in ts: t.join()
print('right:',right)