How to know if an object is still be referenced?

[sohcahtoa82 at gmail.com]

On Wednesday, March 2, 2016 at 3:35:32 AM UTC-8, jf... at ms4.hinet.net wrote:
> 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

"label" might be a local variable, but it's constructor includes a reference to the frame in which it is going.  The frame object will create a reference to the newly-created Label object.  At that point, there will be two references to the new Label object.  When the function exits and "label" goes out of scope, the object still exists because the frame still has a reference.

If you're a C/C++ programmer, another way of thinking of it is imagining EVERY variable is created on the heap, not the stack, and so it is safe to pass around references to "local" variables, even when the function that creates it exits.