[Tutor] Tkinter pass-by-reference query

[Alan Gauld]

> While attempting to add images to a canvas programmatically, I wrote
> the following:
> for i in os.listdir('./icons/terrain'):
> ...  img = PhotoImage(file='./icons/terrain/'+i)
> self.terrainScreen.create_image(int(self.terrainScreen["height"])-50,
> 
> (int(self.terrainScreen["height"])-50),
>                                                  image=img,tag=None)

> with Tkinter in the python shell, and I came to the conclusion that
> the reason nothing was displayed on the canvas was that images are
> passed by reference.

Everything in Python is passed by reference. The issue here is that
one the reference goes out of scope the image object gets garbage
collected!

> In other words, unless there is a permanent `img' variable, the data 
> disappears.

>For example (replace w\ your own gifs):
>>> from Tkinter import *
>>> root = Tk()
>>> terrain = Canvas(root,width=50,height=50)
>>> img = PhotoImage(file='MapEditor/icons/terrain/desert.gif')
>>> terrain.create_image(0,0,image=img)

>Works beautifully, eh?

>>> img = PhotoImage(file='MapEditor/icons/terrain/water.gif')

> Doh! Now it disappears!

You've crated a new image object but you need to pass that to
the canvas, or more easily...
Try configuring the file property of the image:

img.configure(file='MapEditor/icons/terrain/water.gif')

That changes the file used by the image object that is being 
displayed.

The image object is a placeholder within your GUI for displaying
graphics. You only need one object because you are onmly displaying
one image (at a time) in the GUI.

HTH,

Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld