Tkinter or Python issue?

[Eric Brunel]

On Tue, 18 Oct 2005 22:30:33 -0400, Ron Provost <ronpro at cox.net> wrote:

> Hello,
>
> I'm using python 2.4.2 on Win XP Pro.  I'm trying to understand a behavior
> I'm seeing in some Tkinter code I have.  I've reduced my question to a small
> piece of code:
>
>
> #####BEGIN CODE
> #################
> import Tkinter as Tk
> import tkFont
>
> sampleText = """Here is a test string.  This is more text
> Here is a second line of text.  How much
> more can I type.  I can't think of anything else to type.
> """
>
> root = Tk.Tk( )
> t = Tk.Text( root )
> t.pack( )
>
> t.insert( Tk.END, sampleText )
>
> t.tag_config( 'AB', font=tkFont.Font( family='ariel', size=24,
> weight=tkFont.BOLD ) )
> t.tag_config( 'TBU', font=tkFont.Font( family='times', size=10,
> weight=tkFont.BOLD, underline=1 ) )

Here is what I think is happening:
- The first tag_config creates a font using tkFont.Font. At tcl level, this font is not known as an object as in Python, but just as a font name, which is a string. This name happens to be built using the internal identifier for the Python object.
- Once this tag_config is over, no Python variable references your tkFont.Font instance anymore. It is is fact still known at tcl level (via the font name), but Python doesn't know that. So the reference counter for your tkFont.Font instance falls to 0, and the object is discarded.
- The second tag_config seems to create a new font using tkFont.Font. Unfortunately, since the previous font has been discarded, the space occupied by this font is reused for the new font. So the new font happens to have the same internal identifier as the font object created by the first tkFont.Font. So its name is in fact the same as your previous font, and is registered as such at tcl level. So in fact, you didn't create a new font at tcl level, but modified the previous one.

All this actually happens by accident. If you add something allocating some memory between the two tag_config, you'll certainly see your code working. It works when I run it myself...

[snip]
> f1 = font=tkFont.Font( family='ariel', size=24, weight=tkFont.BOLD )
> f2 = font=tkFont.Font( family='times', size=10, weight=tkFont.BOLD,
> underline=1 )
> t.tag_config( 'AB', font=f1 )
> t.tag_config( 'TBU', font=f2 )

You should now see why it works here: your first tkFont.Font is remembered at Python level in a variable. So it is not discarded once the tag_config is over. So the second tkFont.Font is not allocated at the same location, so it doesn't have the same id, and it doesn't have the same name at tcl level. This is the general solution to the problem: keep your fonts in Python variables, so they won't be discarded and their names will never be re-used. You could have written:

f1 = font=tkFont.Font( family='ariel', size=24, weight=tkFont.BOLD )
t.tag_config( 'AB', font=f1 )
f2 = font=tkFont.Font( family='times', size=10, weight=tkFont.BOLD, underline=1 )
t.tag_config( 'TBU', font=f2 )

This should still work. The order is not important; it's just the fact that your fonts are actually known at Python level which prevents Tkinter to reuse their name.

BTW, this is a variant of a well known problem biting newbies regularly, documented here:
http://tkinter.unpythonic.net/wiki/Images

It's basically the same problem for images: it does not suffice to reference an image at tcl level; it must also be referenced at Python level or it will be discarded by Python and you won't be able to use it in your widgets.

HTH
-- 
python -c "print ''.join([chr(154 - ord(c)) for c in 'U(17zX(%,5.zmz5(17;8(%,5.Z65\'*9--56l7+-'])"