python - The root.after() loop cycles twice in one allocated time

Question

I am building a tkinter GUI program which has two loops running together. People have kindly helped me with making "Start" and "Stop" buttons for it (see Creating Tkinter buttons to stop/skip a 2D loop [Solved]). However, a new problem came. In the following code, all the buttons are functioning as we expected. But the speed of the cycling is wrong.

The program is designed to print once per second. However, after finishing printing the first Row, the program starts to print twice per second. I tried a lot but cannot find the reason for that. Can anyone help. Thanks in advance!

import tkinter as tk
root = tk.Tk()

scan_array = [1,"two",3,"ten"]
running = True

Hor_index = 0
Hor_end = 7
Ver_index = 0

def Hor_scan():
    global Hor_index, running, dont_run
    if (Hor_index > Hor_end) and (Ver_index > len(scan_array)):
        running = False
    elif (Hor_index < Hor_end):
        Hor_index += 1
        print ("Row " + str(scan_array[Ver_index-1]) + " Column: " + str(Hor_index))
    else:
        running = False
        Ver_scan()
    if (running):
        root.after (1000, Hor_scan)


def Ver_scan():
    global Ver_index, Hor_index, running
    if Ver_index < len(scan_array):
        Hor_index = 0
        Ver_index += 1
        running = True
        Hor_scan()


def start():
    global Ver_index, Hor_index, running
    Ver_index = 0
    Hor_index = 0
    running = True
    print ("A new run started")
    Ver_scan()

def skip():
    global Ver_index, Hor_index
    Ver_index += 1
    Hor_index = 0

def stop():
    global running
    running = False


btnStart = tk.Button(root,text="Start",command=start)
btnSkip = tk.Button(root,text="Skip",command=skip)
btnStop = tk.Button(root,text="Stop",command=stop)

btnStart.grid()
btnSkip.grid()
btnStop.grid()

root.mainloop()

question from:https://stackoverflow.com/questions/65617700/the-root-after-loop-cycles-twice-in-one-allocated-time

Answer 1

I don't see the point of having an extra if (running): statement in Hor_scan, because this

elif (Hor_index < Hor_end):

condition will anyway be only performed if it is "running". You can make the following changes

def Hor_scan():
    global Hor_index, running, dont_run
    if (Hor_index > Hor_end) and (Ver_index > len(scan_array)):
        running = False
    elif (Hor_index < Hor_end):
        Hor_index += 1
        print ("Row " + str(scan_array[Ver_index-1]) + " Column: " + str(Hor_index))
        root.after (1000, Hor_scan)
    else:
        running = False
        Ver_scan()

REASON : If you you call root.after from the condition as you were doing previously, you will end up scheduling one extra call per cycle, because running does not become False right after you are done with the cycle, it initiates another one, only then your program executes else which actually makes running False.