Tkinter variable trace problem

[C Martin]

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

>



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