CODEWITHSUNDEEP
pythontkinterlabel
Buteman
Buteman

Reputation: 63

label.configure works sometimes why?

Part of my code is as follows:

def get_songs():
    label6.configure(text='Wait')
    os.system('/home/norman/my-startups/grabsongs')
    label6.configure(text='Done')

The label is not updated at the first .configure() but is at the second one. Except if I cause a deliberate error immediately after the first one at which point it is updated and then the program terminates. The system call takes about 2 minutes to complete so it isn't as if there isn't time to display the first one.

I am using Python 2.7.6

Does anyone know why please?

Upvotes: 2

Views: 1591

Answers (1)

Joe Kington
Joe Kington

Reputation: 284890

I'm going to guess you're using Tkinter. If so, as @albert just suggested, you'll want to call label.update_idletasks() or label.update() to tell Tkinter to refresh the display.

As a very crude example to reproduce your problem, let's make a program that will:

  1. Wait 1 second
  2. Do something (sleep for 2 seconds) and update the text to "wait"
  3. Display "done" afterwards

For example:

import Tkinter as tk
import time

root = tk.Tk()
label = tk.Label(root, text='Not waiting yet')
label.pack()

def do_stuff():
    label.configure(text='Wait')
    time.sleep(2)
    label.configure(text='Done')

label.after(1000, do_stuff)
tk.mainloop()

Notice that "Wait" will never be displayed.

To fix that, let's call update_idletasks() after initially setting the text:

import Tkinter as tk
import time

root = tk.Tk()
label = tk.Label(root, text='Not waiting yet')
label.pack()

def do_stuff():
    label.configure(text='Wait')
    label.update_idletasks()
    time.sleep(2)
    label.configure(text='Done')

label.after(1000, do_stuff)
tk.mainloop()

As far as why this happens, it actually is because Tkinter doesn't have time to update the label.

Calling configure doesn't automatically force a refresh of the display, it just queues one the next time things are idle. Because you immediately call something that will halt execution of the mainloop (calling an executable and forcing python to halt until it finishes), Tkinter never gets a chance to process the changes to the label.

Notice that while the gui displays "Wait" (while your process/sleep is running) it won't respond to resizing, etc. Python has halted execution until the other process finishes running.

To get around this, consider using subprocess.Popen (or something similar) instead of os.system. You'll then need to perodically poll the returned pipe to see if the subprocess has finished.

As an example (I'm also moving this into a class to keep the scoping from getting excessively confusing):

import Tkinter as tk
import subprocess

class Application(object):
    def __init__(self, parent):
        self.parent = parent
        self.label = tk.Label(parent, text='Not waiting yet')
        self.label.pack()
        self.parent.after(1000, self.do_stuff)

    def do_stuff(self):
        self.label.configure(text='Wait')
        self._pipe = subprocess.Popen(['/bin/sleep', '2'])
        self.poll()

    def poll(self):
        if self._pipe.poll() is None:
            self.label.after(100, self.poll)
        else:
            self.label.configure(text='Done')

root = tk.Tk()
app = Application(root)
tk.mainloop()

The key difference here is that we can resize/move/interact with the window while we're waiting for the external process to finish. Also note that we never needed to call update_idletasks/update, as Tkinter now does have idle time to update the display.

Upvotes: 3

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