A question about tkinter update

09 May, 2018

Feng writes:

I bought a chinese version of your book. when i tried the paddle ball at this step: 'Bounce (example 4) - ball moving up and down' at the first part of the code, you add a line tk.update(). I cannot see why this is necessary, since there are no balls moving at all. Then, i comment it with a '#'.

Now, it comes with a problem, i failed to see the ball move down, it seems stuck at the top line of the canvas. I tried to print the variable 'pos', and it shows as follows:
------------------(some part of the outcome)
[255.0, 1.0, 270.0, 16.0]
[255.0, 0.0, 270.0, 15.0]
[255.0, -1.0, 270.0, 14.0]
how did this happen, why the first 'tk.update()' cannot be omitted?

The first call to tk.update() isn't strictly necessary - but what it does do is to force a "wait" until all the Tk user interface events have been processed.

When you call pack to tell your Tk components to resize and display themselves in the right place, you're actually sending a message to a whole separate process (this is the Tk event loop). Let's imagine it took 1 minute for each component to set itself up, and you had 10 components in the window. If you called pack it wouldn't take 10 minutes to complete that function call -- it would actually complete almost instantly. Behind the scenes however, the components are busy re-arranging themselves to fit in the window (and in this imaginary scenario, that process would then take 10 minutes to complete).

Try running this code (create a file and run it in IDLE):

from tkinter import *
tk = Tk()
canvas = Canvas(tk, width=500, height=400, bd=0, highlightthickness=0)
canvas.pack()
print(canvas.winfo_height())

You'll get 1. That's because the canvas hasn't finished rearranging itself by the time you call winfo_height. Now try typing the first 4 lines of the code into the IDLE shell -- wait a few seconds then enter the last line:

print(canvas.winfo_height())

Now you'll get 400 (i.e. 400 pixels high). Because you've given the canvas time to complete.

In summary, pack is saying "ok Tk components, re-arrange yourselves on the screen" and update is saying: "ok Tk, I want to wait for that to complete". Which is why, if you remove the update call, inside the __init__ function of the Ball class, it calculates a height of 1, and the code no longer works correctly.