How do I update images on a Tkinter Canvas?

Question

I'm trying to make a script which will enable me to dynamically update an image object and then post the updated image to a Tkinter Canvas widget. The code here is prototype code, just to get the basics down. The aim here is to put a blue pixel on the image being displayed by the canvas, at the click location.

Something very strange is going on here. I'm using the Wing IDE, and if I run this code through the debugger, with a breakpoint at any line in the woohoo function, and then continue execution after hitting the breakpoint, the code works exactly as expected- putting a blue pixel on the image. If I run the code normally, or through the debugger with no breakpoints, the image is never updated. This leads me to the conclusion that there is some internal wizardry going on which I haven't got much hope of understanding without aid.

I'd really like to know the best way to go about this (or any way, I guess), and if someone could explain to me what's going on under the hood that'd be really cool. Thanks.

from Tkinter import *
from PIL import Image, ImageTk

def woohoo(event):

    original.putpixel((event.x,event.y),(0,0,255))

    newpic = ImageTk.PhotoImage(original)
    c.create_image((0,0),image=newpic, anchor="nw")


main = Tk()
c = Canvas(main, width=300, height=300)
main.geometry("300x300+0+0")
c.pack()

original = Image.open("asc.bmp")
picture = ImageTk.PhotoImage(original)
c.create_image((0,0),image=picture, anchor="nw")

c.bind("<Button-1>", woohoo)

main.mainloop()

Answer 1

My guess is, you're creating a new image in a function. The reference to the image is a local variable. When the function exits, the reference is garbage collected which causes the new image to be destroyed. Most likely, running interactively causes the garbage collector to run differently (perhaps more lazily?)

Answer 2

Changed a little of the other post to work with Python 3+ :

from tkinter import *

def stuff(event):
    global picture3
    picture3 = PhotoImage(file='picture2.png')
    c.itemconfigure(picture2, image = picture3)

main = Tk()
c = Canvas(main, width=300, height=300)
c.pack()

picture = PhotoImage(file='picture1.png')
picture2 = c.create_image(150,150,image=picture)

c.bind("<Button-1>", stuff)

main.mainloop()