Is the += operator thread-safe in Python?

Question

I want to create a non-thread-safe chunk of code for experimentation, and those are the functions that 2 threads are going to call.

c = 0

def increment():
          


        

Answer 1

Single opcodes are thread-safe because of the GIL but nothing else:

import time
class something(object):
    def __init__(self,c):
        self.c=c
    def inc(self):
        new = self.c+1 
        # if the thread is interrupted by another inc() call its result is wrong
        time.sleep(0.001) # sleep makes the os continue another thread
        self.c = new


x = something(0)
import threading

for _ in range(10000):
    threading.Thread(target=x.inc).start()

print x.c # ~900 here, instead of 10000

Every resource shared by multiple threads must have a lock.

Answer 2

It's easy to prove that your code is not thread safe. You can increase the likelyhood of seeing the race condition by using a sleep in the critical parts (this simply simulates a slow CPU). However if you run the code for long enough you should see the race condition eventually regardless.

from time import sleep
c = 0

def increment():
  global c
  c_ = c
  sleep(0.1)
  c = c_ + 1

def decrement():
  global c
  c_ = c
  sleep(0.1)
  c  = c_ - 1