Question

I’m having a problem stopping the ‘feed’; the cancel argument doesn’t seem to have any impact on the after method. Although “feed stopped” is printed to the console.

I’m attempting to have one button that will start the feed and another that will stop the feed.

from Tkinter import Tk, Button
import random

    def goodbye_world():
        print "Stopping Feed"
        button.configure(text = "Start Feed", command=hello_world)
        print_sleep(True)

    def hello_world():
        print "Starting Feed"
        button.configure(text = "Stop Feed", command=goodbye_world)
        print_sleep()

    def print_sleep(cancel=False):
        if cancel==False:
            foo = random.randint(4000,7500)
            print "Sleeping", foo
            root.after(foo,print_sleep)
        else:
            print "Feed Stopped"


    root = Tk()
    button = Button(root, text="Start Feed", command=hello_world)

    button.pack()


    root.mainloop()

With the output:

Starting Feed
Sleeping 4195
Sleeping 4634
Sleeping 6591
Sleeping 7074
Stopping Feed
Sleeping 4908
Feed Stopped
Sleeping 6892
Sleeping 5605

Answer 1

The problem is that, even though you’re calling print_sleep with True to stop the cycle, there’s already a pending job waiting to fire. Pressing the stop button won’t cause a new job to fire but the old job is still there, and when it calls itself, it passes in False which causes the loop to continue.

You need to cancel the pending job so that it doesn’t run. For example:

def cancel():
    if self._job is not None:
        root.after_cancel(self._job)
        self._job = None

def goodbye_world():
    print "Stopping Feed"
    cancel()
    button.configure(text = "Start Feed", command=hello_world)

def hello_world():
    print "Starting Feed"
    button.configure(text = "Stop Feed", command=goodbye_world)
    print_sleep()

def print_sleep():
    foo = random.randint(4000,7500)
    print "Sleeping", foo
    self._job = root.after(foo,print_sleep)

Note: make sure you initialize self._job somewhere, such as in the constructor of your application object.