Error in 2.2.1 Tkinter? (was: limiting text input in Tkinter Entry widget)

[Cameron Laird]

In article <103ho4p6m98gs97 at news.supernews.com>,
John Roth <newsgroups at jhrothjr.com> wrote:
>
>"Otto Krüse" <otto_kruse at hotmail.com> wrote in message
>news:4038d6c2$0$49821$1b2cd167 at news.wanadoo.nl...
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>> Does anyone know a way to limit the amount of characters an entry widget
>> can take? Is there an easy option to set for this or does this problem
>> require some python code? Can't seem to find answers in any documentation.
>>
>> The code:
>> self.e1 = Entry(frame, width="4")
>> self.e2 = Entry(frame, width="2")
>
>The MegaWidgits package has an EntryField widgit that has some
>built-in validation, and has a hook for you to insert a validation function
>or method.
>
>Otherwise, you need to do the validation as you collect text characters
>and pass them to the widgit.
>
>Validation code is inherently ugly. Not complex, just ugly. In your case
>I'd probably do some form of pattern driven validation.
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Without disputing the general proposition about the aesthetics
of validation of user data, there's supposed to be a simpler 
answer.  Tkinter includes textvariable and trace capabilities
which should make this as compact as
  import Tkinter

  root = Tkinter.Tk()

  def validate(name, index, mode):
    value = root.getvar(name)
      # Truncate the entry text to its first four characters.
    root.setvar(name, value[0:4])

  my_variable = Tkinter.StringVar()
  my_variable.trace_variable('w', validate)

  my_entry = Tkinter.Entry(width = 4, textvariable = my_variable)
  my_entry.pack()

  Tkinter.mainloop
In practice, one would likely encapsulate the "bookkeeping" in
an object.

The problem is that, when I run this example, Tkinter tosses the
exception
  TclError: can't read "PY_VAR0": no such variable
This surprises me.  I'm *sure* I've done this before.  Before I
dive into the distribution source code, is it obvious to anyone
else what I'm missing?
-- 

Cameron Laird <claird at phaseit.net>
Business:  http://www.Phaseit.net