[Python-bugs-list] [ python-Bugs-531262 ] Wrong result for Canvas.tag_bind(t, s)

[noreply@sourceforge.net]

Bugs item #531262, was opened at 2002-03-18 09:07
You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=531262&group_id=5470

Category: Tkinter
Group: Python 2.1.1
Status: Open
Resolution: None
Priority: 5
Submitted By: Eric Brunel (ebrunel)
Assigned to: Nobody/Anonymous (nobody)
Summary: Wrong result for Canvas.tag_bind(t, s)

Initial Comment:
When a binding is defined for a tag in a Tkinter 
Canvas, trying to get the callback using the 
"tag_bind" method with 2 parameters returns a useless 
value:

----------------------------
from Tkinter import *

root = Tk()

## Create the canvas
c = Canvas(root)
c.pack()

## Create the item
tId = c.create_text(100, 100, text='spam')

## Create the binding
def foo(event): print 'bar'
c.tag_bind(tId, '<Button-1>', foo)

## Get and print the binding
print c.tag_bind(tId, '<Button-1>')

root.mainloop()
---------------------------

The programs prints something looking like: 'if 
{"[136128196foo %# %b %f %h %k %s %t %w %x %y %A %E 
%K %N %W %T %X %Y %D]" == "break"} break'. Trying to 
pass this value as the third parameter of tag_unbind 
for example results in a TclError:

Traceback (most recent call last):
  File "tkCanvasBindings2.py", line 16, in ?
    c.tag_unbind(tId, '<Button-1>', f)
  File "/usr/local/lib/python2.1/lib-tk/Tkinter.py", 
line 1904, in 
tag_unbind
    self.deletecommand(funcid)
  File "/usr/local/lib/python2.1/lib-tk/Tkinter.py", 
line 297, in 
deletecommand
    self.tk.deletecommand(name)
TclError: can't delete Tcl command

This happens apparently on all platforms (tried 
Win2K, Linux Mandrake 8.0 and Solaris 2.7).



----------------------------------------------------------------------

>Comment By: Eric Brunel (ebrunel)
Date: 2002-03-22 17:12

Message:
Logged In: YES 
user_id=489050

In fact, I saw this problem when trying to delete the 
bindings made for a Canvas: I noticed that when refreshing 
several times a canvas by deleting, then recreating all 
the items (including the bindings), the occupied memory 
increased with no apparent reason. So I dived into 
Tkinter.py, and found out that Tcl commands were created 
for each binding, but were only deleted either when 
deleting the Canvas (which I couldn't do), or when calling 
the tag_unbind method with the tagOrId, the sequence *and* 
the Tcl command name. So I tried to get this command name 
by calling tag_bind, and saw that the result was not what 
I expected.
The solution to remember the result of the call to 
tag_bind creating the binding provides a solution to this 
problem. It may just be a bit strange to explicitely 
remember the bindings in the application, since Tk/Tkinter 
already remembers them. It may also seem strange to get 
via tag_bind something that is completely useless at the 
Python level...

Anyway, thanks a lot.


----------------------------------------------------------------------

Comment By: A.M. Kuchling (akuchling)
Date: 2002-03-22 16:44

Message:
Logged In: YES 
user_id=11375

I don't think this is actually a bug, and the return 
value of tag_bind really is useless.  tag_bind(t,s) 
returns the command bound to that sequence.  
Tkinter.py supports a feature where a binding can 
return the string "break" if no further bound function 
should be invoked, so when adding a binding, Python has 
to synthesize the 'if {123456foo ...}' Tcl command, and 
that's what tag_bind returns.

I think your example code should be written as:

bind_id = c.tag_bind(tId, '<Button-1>', foo)

And then refer to bind_id when you need to delete the 
binding.  

Was your example usage of tag_bind() described in some 
documentation, or in a demo script somewhere?  
Maybe that script or documentation needs to be fixed.


----------------------------------------------------------------------

You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=531262&group_id=5470