multithreading - Python Tkinter Label redrawing every 10 seconds

Question

I have this following Python Tkinter code which redraw the label every 10 second. My question is , to me it seems like it is drawing the new label over and over again over the old label. So, eventually, after a few hours there will be hundreds of drawing overlapping (at least from what i understand). Will this use more memory or cause problem?

import Tkinter as tk
import threading

def Draw():
    frame=tk.Frame(root,width=100,height=100,relief='solid',bd=1)
    frame.place(x=10,y=10)
    text=tk.Label(frame,text='HELLO')
    text.pack()

def Refresher():
    print 'refreshing'
    Draw()
    threading.Timer(10, Refresher).start()

root=tk.Tk()
Refresher()
root.mainloop()

Here in my example, i am just using a single label.I am aware that i can use textvariable to update the text of the label or even text.config. But what am actually doing is to refresh a grid of label(like a table)+buttons and stuffs to match with the latest data available.

From my beginner understanding, if i wrote this Draw() function as class, i can destroy the frame by using frame.destroy whenever i execute Refresher function. But the code i currently have is written in functions without class ( i don't wish to rewrite the whole code into class).

The other option i can think of is to declare frame in the Draw() as global and use frame.destroy() ( which i reluctant to do as this could cause name conflict if i have too many frames (which i do))

If overdrawing over the old drawing doesn't cause any problem (except that i can't see the old drawing), i can live with that.

These are all just my beginner thoughts. Should i destroy the frame before redraw the updated one? if so, in what way should i destroy it if the code structure is just like in my sample code? Or overdrawing the old label is fine?

EDIT

Someone mentioned that python tkinter is not thread safe and my code will likely to fail randomly.

I actually took this link as a reference to use threading as my solution and i didn't find anything about thread safety in that post.

I am wondering what are the general cases that i should not use threading and what are the general cases i could use threading?

Answer 1

The correct way to run a function or update a label in tkinter is to use the after method. This puts an event on the event queue to be executed at some time in the future. If you have a function that does some work, then puts itself back on the event queue, you have created something that will run forever.

Here's a quick example based on your example:

import Tkinter as tk
import time

def Draw():
    global text

    frame=tk.Frame(root,width=100,height=100,relief='solid',bd=1)
    frame.place(x=10,y=10)
    text=tk.Label(frame,text='HELLO')
    text.pack()

def Refresher():
    global text
    text.configure(text=time.asctime())
    root.after(1000, Refresher) # every second...

root=tk.Tk()
Draw()
Refresher()
root.mainloop()

There are a lot of things I would change about that program from a coding style point of view, but I wanted to keep it as close to your original question as possible. The point is, you can use after to call a function that updates the label without having to create new labels. Plus, that function can arrange for itself to be called again at some interval. In this example I picked one second just so that the effect is easier to see.

You also asked "I am wondering what are the general cases that i should not use threading and what are the general cases i could use threading?"

To put a blunt point on it, you should never use threading if you have to ask a question about when to use threading. Threading is an advanced technique, it is complicated, and it easy to get wrong. It's quite possible for threading to make your program slower rather than faster. It has subtle consequences, such as your programs failing mysteriously if you do things that aren't thread safe.

To be more specific to your situation: you should avoid using threads with tkinter. You can use them, but you can't access widgets from these other threads. If you need to do something with a widget, you must put an instruction in a thread-safe queue, and then in the main thread you need to periodically check that queue to see if there's an instruction to be processed. There are examples of that on this website if you search for them.

If all that sounds complicated, it is. For most of the GUIs you write, you won't need to worry about that. If you can take large processes and break them down into chunks that execute in 100 ms or less, you only need after, and never need threads.