multi.py

# -*- coding: utf-8 -*-
"""
Since we have GIL, should we add Lock for global variable read/write?
"""

import threading


shared_resource_with_no_lock = 0
COUNT = 100000


def increment_without_lock():
    global shared_resource_with_no_lock
    for i in range(COUNT):
        shared_resource_with_no_lock += 1


def decrement_without_lock():
    global shared_resource_with_no_lock
    for i in range(COUNT):
        shared_resource_with_no_lock -= 1


while 1:
    t3 = threading.Thread(target=increment_without_lock)
    t4 = threading.Thread(target=decrement_without_lock)
    t3.start()
    t4.start()
    t3.join()
    t4.join()
    print(shared_resource_with_no_lock)

If you waiting long enough you will see same problems happens in Python3

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^CTraceback (most recent call last):
  File "multi.py", line 31, in <module>
    t4.join()
  File "/usr/local/var/pyenv/versions/3.7.0/lib/python3.7/threading.py", line 1032, in join
    self._wait_for_tstate_lock()
  File "/usr/local/var/pyenv/versions/3.7.0/lib/python3.7/threading.py", line 1048, in _wait_for_tstate_lock
    elif lock.acquire(block, timeout):
KeyboardInterrupt
➜