Running a loop alongside a Tkinter application

Question

I am a high school programming student and I have a small question. I have been tasked with writing a simple game in Tkinter where an icicle falls from the ceiling and you have to avoid it with your mouse. Simple enough. However, I have hit an issue. Whenever I run a loop in a Tkinter application, it won't open. I've tried with a for loop that pauses every .5 seconds using time.sleep() and the window opens as soon as the loop finishes. Is there some special thing I need to do to make loops work in Tkinter?

from Tkinter import *
import time
import random

class App:
    def __init__(self, parent):
        self.frame = Frame(root, bg= '#1987DF', width=800, height=800)
        self.frame.bind("<Motion>", self.motionevent)
        self.frame.pack()
        #self.run()
    def randhex(self):
        b = "#"
        for i in range(1, 7):
            a = random.randint(0, 15)
            if a == 10:
                a = "A"
            elif a == 11:
                a = "B"
            elif a == 12:
                a = "C"
            elif a == 13:
                a = "D"
            elif a == 14:
                a = "E"
            elif a == 15:
                a = "F"
            b = b+str(a)
        return b

    def motionevent(self, event):
        xpos, ypos, bg = event.x, event.y, self.randhex()
        str1 = "X : %d  Y : %d BG : %s" % (xpos, ypos, bg)
        root.title(str1)
        x,y, delta = 100, 100, 10
        self.frame.config(bg=bg)

    def run(self):
        for i in range(0, 10):
            time.sleep(.5)
            print 'i'
            self.frame.config(bg=self.randhex())

root = Tk()
app = App(root)
root.mainloop()

Currently all it is supposed to do is change the background when the mouse moves. When the line in init that says self.run() is uncommented it will print 'i' 10 times then the window will open. Help?

Answer 1

Writing an event based program is not the same as writing traditional programs. There is already an infinite loop running, and like any infinite loop, if you place another loop inside, the outer loop can't continue until the inner loop finishes. Since the outer loop is what causes the screen to refresh and events to be processed, inner loops effectively freeze your app until they are done.

Since there is already a loop running you don't need to create another loop. All you need to do is add little jobs to the event queue one at a time. Each job is, in effect, one iteration of your inner loop.

For example, if you wanted to write an inner loop like this:

for i in range(10):
    print "i:", i

... you would instead add an event to the event queue and each time the event loop iterates (or more precisely, each time it finishes processing any other events) it will do one iteration of your loop. You do it like this:

def do_one_iteration(i):
    print "i:", i
    if i < 9:
        root.after_idle(do_one_iteration, i+1)

Then, after the first time you call do_one_iteration, it will place the next iteration of itself on the event queue, and continue to do so until it decides it is done.

Typically you would call do_one_iteration when the user presses a button (eg: the "start" button). Call it once, then it does one bit of work (ie: moving the icicle down a couple of pixels) and then reschedules itself.

In game development you might have a function called update_display which is in charge of redrawing everything. It could, for example, subtract 1 from the Y coordinate of each icicle. You can add bindings for the left and right arrows to move the player (by incrementing or decrementing the X coordinate), and your update function would use these new coordinates to redraw the player.

By the way, you can slow down your program by using after instead of after_idle to call the function after a slight delay. This delay can be used to control the frame rate. For example, assuming the update is nearly instantaneous (and it probably will be in your case), calling after with an argument of 41 (milliseconds) yields a framerate of approximately 24 fps (24 frames times 41 milliseconds equals 984 milliseconds, or roughly 24 frames per second)

It may sound complicated but it's really pretty easy in practice once you do it once or twice.