Using a global variable with a thread

Question

How do I share a global variable with thread?

My Python code example is:

from threading import Thread
import time
a = 0  #global variable

def thread         


        

Answer 1

Well, running example:

WARNING! NEVER DO THIS AT HOME/WORK! Only in classroom ;)

Use semaphores, shared variables, etc. to avoid rush conditions.

from threading import Thread
import time

a = 0  # global variable


def thread1(threadname):
    global a
    for k in range(100):
        print("{} {}".format(threadname, a))
        time.sleep(0.1)
        if k == 5:
            a += 100


def thread2(threadname):
    global a
    for k in range(10):
        a += 1
        time.sleep(0.2)


thread1 = Thread(target=thread1, args=("Thread-1",))
thread2 = Thread(target=thread2, args=("Thread-2",))

thread1.start()
thread2.start()

thread1.join()
thread2.join()

and the output:

Thread-1 0
Thread-1 1
Thread-1 2
Thread-1 2
Thread-1 3
Thread-1 3
Thread-1 104
Thread-1 104
Thread-1 105
Thread-1 105
Thread-1 106
Thread-1 106
Thread-1 107
Thread-1 107
Thread-1 108
Thread-1 108
Thread-1 109
Thread-1 109
Thread-1 110
Thread-1 110
Thread-1 110
Thread-1 110
Thread-1 110
Thread-1 110
Thread-1 110
Thread-1 110

If the timing were right, the a += 100 operation would be skipped:

Processor executes at T a+100 and gets 104. But it stops, and jumps to next thread Here, At T+1 executes a+1 with old value of a, a == 4. So it computes 5. Jump back (at T+2), thread 1, and write a=104 in memory. Now back at thread 2, time is T+3 and write a=5 in memory. Voila! The next print instruction will print 5 instead of 104.

VERY nasty bug to be reproduced and caught.