Re: How to know if an object is still be referenced?

Path	csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From	Terry Reedy <tjreedy@udel.edu>
Newsgroups	comp.lang.python
Subject	Re: How to know if an object is still be referenced?
Date	Wed, 2 Mar 2016 02:03:33 -0500
Lines	44
Message-ID	<mailman.95.1456902233.20602.python-list@python.org> (permalink)
References	<8e9f1a84-cced-435e-a379-e1e2ac03f483@googlegroups.com>
Mime-Version	1.0
Content-Type	text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding	7bit
X-Trace	news.uni-berlin.de VDdfeoCkmsXt+C4RzQvixw/Nllbf3YSmFvkajMvfUZxw==
Return-Path	<python-python-list@m.gmane.org>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.003
X-Spam-Evidence	'H': 0.99; 'S': 0.00; 'exit': 0.07; 'rewrite': 0.07; 'subject:How': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'subject:still': 0.09; 'jan': 0.11; 'def': 0.13; 'explicitly': 0.15; 'received:80.91.229.3': 0.16; 'received:io': 0.16; 'received:plane.gmane.org': 0.16; 'received:psf.io': 0.16; 'redundancy': 0.16; 'reedy': 0.16; 'subject:object': 0.16; 'suggestion.': 0.16; 'tk()': 0.16; 'wrote:': 0.16; 'attribute': 0.18; 'correctly.': 0.22; 'tkinter': 0.22; 'second': 0.24; 'import': 0.24; 'header:In-Reply-To:1': 0.24; 'header:User-Agent:1': 0.26; 'header:X-Complaints-To:1': 0.26; 'then.': 0.27; 'function': 0.28; 'referenced': 0.29; 'work.': 0.30; 'another': 0.32; 'noticed': 0.32; 'class': 0.33; 'label': 0.35; 'question,': 0.35; 'problem.': 0.35; 'skip:p 30': 0.35; 'but': 0.36; 'to:addr:python-list': 0.36; 'subject:?': 0.36; 'pm,': 0.36; 'subject:: ': 0.37; 'two': 0.37; 'display': 0.37; 'received:org': 0.37; 'delete': 0.38; 'skip:p 20': 0.38; 'to:addr:python.org': 0.40; 'received:96': 0.63; 'times': 0.63; 'binding': 0.66; 'here': 0.66; 'worth': 0.67; 'picture': 0.70; 'saving': 0.70; 'photo': 0.81; 'disappears': 0.84; 'received:fios.verizon.net': 0.91; 'subject:know': 0.91
X-Injected-Via-Gmane	http://gmane.org/
X-Gmane-NNTP-Posting-Host	pool-96-227-207-81.phlapa.fios.verizon.net
User-Agent	Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0
In-Reply-To	<8e9f1a84-cced-435e-a379-e1e2ac03f483@googlegroups.com>
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.21
Precedence	list
List-Id	General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe	<https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive	<http://mail.python.org/pipermail/python-list/>
List-Post	<mailto:python-list@python.org>
List-Help	<mailto:python-list-request@python.org?subject=help>
List-Subscribe	<https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Xref	csiph.com comp.lang.python:103843

Show key headers only | View raw

On 3/1/2016 9:35 PM, jfong@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

Thread

How to know if an object is still be referenced? jfong@ms4.hinet.net - 2016-03-01 18:35 -0800
  Re: How to know if an object is still be referenced? Terry Reedy <tjreedy@udel.edu> - 2016-03-02 02:03 -0500
    Re: How to know if an object is still be referenced? jfong@ms4.hinet.net - 2016-03-02 03:35 -0800
      Re: How to know if an object is still be referenced? sohcahtoa82@gmail.com - 2016-03-02 15:38 -0800
        Re: How to know if an object is still be referenced? jfong@ms4.hinet.net - 2016-03-02 18:36 -0800

csiph-web