Inconsistent counting / timing with threading

[rantwhy]

I have made a counter using threading I'm not sure why when the tick thread is flipping between 1 and 0 it is not consistent with its timing. Also with my incremental counter counting the changes of the other thread it changes the value in clumps.

import threading
import time

countval = 0
def tick():
    global countval
    while True:
        countval = 1
        time.sleep(1)
        countval = 0
        time.sleep(1)
        print ("clock value: " + str(countval))

def increment_value():
    global countval
    plus = 0
    flip = 1
    while True:
        if countval == flip:
            plus = plus + 1
            flip = 0
            print ("count up value: " + str(plus))
        if countval == 0:
            flip = 1

incr_thread = threading.Thread(target=increment_value)
tick = threading.Thread(target=tick)
incr_thread.start()
tick.start()

Here's the output I get what would cause it to be so inconsistent?
I would expect it to output count up value then clock value and so on but it sometimes puts 2 count up values.

count up value: 1
clock value: 0
count up value: 2
count up value: 3
clock value: 0
count up value: 4
count up value: 5
clock value: 0
count up value: 6
count up value: 7
clock value: 0
count up value: 8
clock value: 0
count up value: 9
clock value: 0
count up value: 10
count up value: 11
clock value: 0
count up value: 12
clock value: 0
count up value: 13
count up value: 14
clock value: 0
count up value: 15
count up value: 16

[deanhystad]

I would expect something like this:

import threading
import time
 
toggle = 0

def tick():
    global toggle
    while True:
        time.sleep(1)
        toggle = 0 if toggle == 1 else 1
        print ("toggle:", toggle)
 
def increment_value():
    last = toggle
    count = 0
    while True:
        if last != toggle:
            last = 0 if last == 1 else 1
            count += 1
            print ("count:", count)
 
incr_thread = threading.Thread(target=increment_value)
tick = threading.Thread(target=tick)
incr_thread.start()
tick.start()

As far as increment_value() is concerned, countval can change at any time. Your code behaves differently depending on where it is in increment_value() when countval changes.

def increment_value():
    global countval
    plus = 0
    flip = 1
    while True:
        if countval == flip:  # If countval changes 1->0 here, flip == countval so plus is incremented
            plus = plus + 1
            flip = 0
            print ("count up value: " + str(plus))
        if countval == 0:   # If countval changes 1->0 here, flip is set to 1 and we don't count the tick
            flip = 1