Tkinter variable trace problem

[Peter Otten]

C Martin wrote:

> What am I doing wrong in this code? The callback doesn't work from the Entry widget.
> 
> ##start code
> import Tkinter
>   
> tk = Tkinter.Tk()
> var = Tkinter.StringVar()
> print var._name
> 
> def cb(name, index, mode):
>   print "callback called with name=%r, index=%r, mode=%r" % (name, index, mode)
>   varValue = tk.getvar(name)
>   print "    and variable value = %r" % varValue
> 
> var.trace('w', cb)
> 
> var.set('test')
> entry = Tkinter.Entry(tk, textvariable=var)
> entry.pack()
> 
> tk.mainloop()
> ##end code
> 
> It produces the following output. The first three lines appear right away, and the exception occurs when you type in the entry widget:
> 
>>test.py
> PY_VAR0
> callback called with name='PY_VAR0', index='', mode='w'
>     and variable value = 'test'
> callback called with name='PY_VAR0', index='', mode='w'
> Exception in Tkinter callback
> Traceback (most recent call last):
>   File "C:\Python25\lib\lib-tk\Tkinter.py", line 1403, in __call__
>     return self.func(*args)
>   File "D:\APCC\Projects\Utilities\VisualData\test.py", line 9, in cb
>     varValue = tk.getvar(name)
>   File "C:\Python25\lib\lib-tk\Tkinter.py", line 421, in getvar
>     return self.tk.getvar(name)
> TclError: can't read "PY_VAR0": no such variable

A quick look in the Tkinter source reveals that you need
tk.globalgetvar(), not tk.getvar(). 

I would actually recommend var.get(), so that you don't have to mess with
these internals at all.

Peter