X-Received: by 10.140.159.7 with SMTP id f7mr141562qhf.3.1456886110885; Tue, 01 Mar 2016 18:35:10 -0800 (PST)
X-Received: by 10.50.29.108 with SMTP id j12mr72257igh.1.1456886110425; Tue, 01 Mar 2016 18:35:10 -0800 (PST)
Path: csiph.com!au2pb.net!usenet.blueworldhosting.com!feeder01.blueworldhosting.com!peer02.iad.highwinds-media.com!news.highwinds-media.com!feed-me.highwinds-media.com!y89no5046336qge.0!news-out.google.com!k1ni6017igd.0!nntp.google.com!ok5no5624403igc.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.python
Date: Tue, 1 Mar 2016 18:35:10 -0800 (PST)
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=36.238.103.179; posting-account=G2sM6AoAAADOlDdo9rWD6sFkj3T5ULsz
NNTP-Posting-Host: 36.238.103.179
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <8e9f1a84-cced-435e-a379-e1e2ac03f483@googlegroups.com>
Subject: How to know if an object is still be referenced?
From: jfong@ms4.hinet.net
Injection-Date: Wed, 02 Mar 2016 02:35:10 +0000
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
X-Received-Bytes: 2625
X-Received-Body-CRC: 1454408937
Xref: csiph.com comp.lang.python:103834

Recently I was puzzled by a tkinter problem. The codes below (from a book) =
can display the picture correctly.

    gifdir =3D "../gifs/"
    from tkinter import *
    win =3D Tk()
    photo =3D PhotoImage(file=3Dgifdir + "ora-pp.gif")
    Button(win, image=3Dphoto).pack()
    win.mainloop()

And the codes below (from another book) will also work.

    class DrumMachine:
        ....
        ....
        def create_play_bar(self):
            ....
            ....
            photo =3D PhotoImage(file=3D'images/signature.gif')
            label =3D Label(playbar_frame, image=3Dphoto)
            label.image =3D photo
            label.grid(row=3Dstart_row, column=3D50, padx=3D1, sticky=3D'w'=
)
        ....
        ....

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 =3D =
photo". But it fails then.

How can it be? one works and the other not.

I search for answers on the web and here are some links talking about it:

http://stackoverflow.com/questions/20812579/why-it-shows-blank-instead-of-p=
icture-in-my-tkinter-program
http://effbot.org/tkinterbook/photoimage.htm

They all said that you have to keep a reference to the "photo" or else it w=
ill be garbage collected. Now, as I had learn Python so far, I know that a =
keyword argument passed in an object instantiation will bind this object to=
 its attribute name, i.e. create a reference to this object. (and it should=
 or the whole language will be in disaster)

Can anyone help me out?