Tkinter - Event Queue and order of events

Question

I tried to figure out how the event queue is processed in tkinter. I found this link once and since I played around but I observed different behavior.

---

The top of my code is:

import tkinter as tk
from tkinter import ttk

def p_trace(*tkargs):
    return print('trace', var.get())
def p_binding(event):
    return print(event.type,var.get())

root = tk.Tk()
var = tk.IntVar(root)
var.trace_add('write',p_trace)

---

The p_after is on different location in the print order by using 0 ms.

def after_done():
    return print('after',var.get())

check3= ttk.Checkbutton(root,text='event/trace/after',
                       command=lambda t=0:root.after(t,after_done),
                       variable=var)
check3.bind('<Button-1>',p_binding)
check3.pack()

The expected order would be for me, based on the link I have found, like this:

1. ButtonPress 0
2. trace 1
3. after 1
4. ButtonPress 1
5. trace 0
6. after 0

But if you click on the button, like gambling, the actual order can be sometimes:

1. ButtonPress 1
2. trace 0
3. ButtonPress 0
4. after 0
5. trace 1
6. after 1

---

The p_pseudo should be always on top via when='head' but it isn't, never:

def p_pseudo(*tkargs):
    return print('pseudo',var.get())

check2= ttk.Checkbutton(root,text='event/trace/pseudo',
                       command=lambda:check2.event_generate('<<pseudo_event>>',when='head'),
                       variable=var)
check2.bind('<Button-1>',p_binding)
check2.bind('<<pseudo_event>>',p_pseudo)
check2.pack()

The p_after with after_idle gives also strange output.

def after_done():
    return print('after',var.get())

check4= ttk.Checkbutton(root,text='event/trace/after',
                       command=lambda:root.after_idle(after_done),
                       variable=var)
check4.bind('<Button-1>',p_binding)
check4.pack()

---

This seems like arbitrary behavior and makes it in some of my cases hard to have the right order of events. I need to slow my program down for secure execution. Is it possible to determinate the event queue of tkinter or its not likely?

Answer 1

To order the Event Queue of tkinter one have either to bypass all events and trigger them in the prefered order. Or works with a specific type of event, means a sepecific flag. Otherwise tkinters Notifier will process the order of events like below. Another solution for people of tcl knowlege would be to write a own Notifier like described here.

Tcl_Notifier
  |
  |
  |

Tcl_DoOneEvent ------+----> get_first/oldest from Tcl_WINDOW_EVENTS (processes msg of WM)
  ^                  |
  |                  +----> get_first/oldest from Tcl_FILE_EVENTS (tk.Tk.createfilehandler)
  |                  |
  |                  +----> get_first/oldest from Tcl_TIMER_EVENTS (after)
  |                  |
  |                  +----> get_first/oldest from Tcl_IDLE_EVENTS (after_idle)
  |                  |
  |                  +----> get_first/oldest from Other event sources (virtual events)
  |                  |
  |                  +----> Tcl_SetTimer(time|maxtime) ----> Tcl_WaitForEvent ----+
  |                                                                               |
  +-------------------------------------------------------------------------------+

BTW, there are some additional conclusions:

• If after maxtime the command Tcl_DoOneEvent cant be invoked there will be a runtime error.