[Tutor] Tkinter pass-by-reference query

[Henry Finucane]

Thanks for your help, I've gotten it to work.

On 5/20/06, Alan Gauld <alan.gauld at freenet.co.uk> wrote:
> > 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
>
>
>


-- 
--H.F.
My penguin is bigger than yours, mister...