timer.py

from time import sleep

hours = 0
minutes = 0
seconds = 0

while True:
    if seconds == 60:
        minutes += 1
        seconds = 0
    if minutes == 60:
        hours += 1
        minutes = 0
    print("{:02d}:{:02d}:{:02d}".format(hours, minutes, seconds), end='\r')
    sleep(1)
    seconds += 1