How to know if an object is still be referenced?

[jfong at ms4.hinet.net]

Terry Reedy at 2016/3/2  UTC+8 3:04:10PM wrote：
> On 3/1/2016 9:35 PM, jfong at ms4.hinet.net wrote:
> > Recently I was puzzled by a tkinter problem. The codes below (from a book) can display the picture correctly.
> >
> >      gifdir = "../gifs/"
> >      from tkinter import *
> >      win = Tk()
> >      photo = PhotoImage(file=gifdir + "ora-pp.gif")
> >      Button(win, image=photo).pack()
> >      win.mainloop()
> 
> Since photo is a global name, the binding remain until you explicitly 
> delete it or exit the app.
> 
> > And the codes below (from another book) will also work.
> >
> >      class DrumMachine:
> >          ....
> >          ....
> >          def create_play_bar(self):
> >              ....
> >              ....
> >              photo = PhotoImage(file='images/signature.gif')
> >              label = Label(playbar_frame, image=photo)
> >              label.image = photo
> >              label.grid(row=start_row, column=50, padx=1, sticky='w')
> >          ....
> >          ....
> 
> Here photo is a local name and the binding disappears when the function 
> exits.  I would rewrite it to follow pattern 1.
> 
>               self.photo = PhotoImage(file='images/signature.gif')
>               label = Label(playbar_frame, image=self.photo)
> 
> To me, saving an attribute reference is not worth the extra line.
> 
> > In the second example, I noticed that the "photo" was referenced two times
> > and I think it might be a redundancy so I remove the line "label.image = photo". But it fails then.
> 
> On another question, I made the same suggestion.  Oops.
> 
> -- 
> Terry Jan Reedy

Thanks, Terry. After reading your reply I noticed that I had make a mistake. The "image" is not an attribute of the Button. It's an option. The "label.image=photo" does a different thing. But it didn't help on solving my puzzle.

If the problem was caused by the "photo" is a local, then binding the "photo" to an local attribute ("label" is a local too) seems no meaning at all. But it did make difference! No idea how the underneath mechanism works.

I run this example under the pdb and it can display the picture correctly even without the "label.image=photo" statement. Why it fails on running at real time? tk event scheduling? I don't know.

--Jach