threading

[ast]

Hi

An operation like x+=1 on a global variable x is not thread safe because 
there can be a thread switch between reading and writing to x.
The correct way is to use a lock

lock = threading.Lock

with lock:
     x+=1

I tried to write a program without the lock which should fail.
Here it is:

import threading

x = 0

def test():
     global x
     for i in range(100):
         x+=1

threadings = []

for i in range(100):
     t = threading.Thread(target=test)
     threadings.append(t)
     t.start()

for t in threadings:
     t.join()

print(x)

10000

The result is always correct: 10000
Why ?

Secondly, how the switch between threads is done by the processor ? Is 
there a hardware interrupt coming from a timer ?