奇怪的行为Python线程

Question

I am unable to understand the behavior of this code.

import sys
import threading
import time
n = 0
e = threading.Event()
# q = False

def foo():
    global n
    while not e.is_set():
        time.sleep(2)
        print("Value ", n)
        n = 0

t = threading.Thread(target=foo)
t.start()

while True:
    try:
        n += 1
        time.sleep(1)
    except KeyboardInterrupt:
        e.set()

Output

Value  2
Value  1
Value  1
Value  2
Value  2
Value  1
Value  2
Value  2
Value  2
Value  1
Value  2
Value  1
Value  2
Value  1
Value  1
Value  1
Value  1
^CValue  3
^C^C^C

When I am typing Ctrl-C for the first time. the program doesn't print anything and gets blocked and doesn't respond to further Ctrl-C.Can somebody explain this behavior

Answer 1

3Think of this, lets say we have the Thread1, Thread2 and value with time(T):

Value = 0 at Time 0
    Value at Thread1 at Time0 is 0, it is incremented so Value = 1
    Value at Thread2 at Time0 is 0 again because is accesed at the same time, it is incremented so Value = 1

Value = 1 at Time 1
    Value at Thread2 at Time1 is 1, it is incremented so Value = 2
    Value at Thread1 at Time1 is 2 (it was incremented before the Thread1 could acces it), it is incremented so Value = 3

As you can see, if you dont handle acceses to resources from the threads they could be accesed at any time, even at the same time where the troubles start.

You are only handling the keyboard interrupt in the main Thread, this cause the hanging, as you are handling it but not terminating the second thread, e is accesed by the 2 threads also so it is still undefined behaviour.