How to restrict lenght of entry widget to certain number of character

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Peter Otten wrote:
> Michael Onfrek wrote:
> 
> 
>>import Tkinter as tk
>>
>>Hi! Can you explain what line above mean?
>>
>>I also found : http://effbot.org/zone/tkinter-entry-validate.htm
>>
>>It works for me, but I not really  understand how? :)
> 
> 
>>>import Tkinter as tk
> 
> 
> Make objects defined in Tkinter available under the tk prefix.
> E. g. to access an Entry you can do 'tk.Entry'. Had you imported it 
> 'import Tkinter' you would have to do 'Tkinter.Entry' instead. So you
> are saving a few keystrokes. Doing 'from Tkinter import *' saves you still
> more keystrokes but is considered bad style except for demonstration
> purposes.
> 
> 
>>>var = tk.StringVar()
>>>entry = tk.Entry(root, textvariable=var)
> 
> 
> Create a StringVar and connect it to the Entry widget. Any changes the user
> makes in the Entry are reflected in the StringVar's value which can be
> accessed with its get() method.
> 
> 
>>>max_len = 5
>>>def on_write(*args):
>>>    s = var.get()
>>>    if len(s) > max_len:
>>>        var.set(s[:max_len])
> 
> 
> Define a function that doesn't care about the arguments passed to it. It 
> reads the current value of the StringVar 'var' and, if necessary, trims it
> to 'max_len_' characters.
> 
> 
>>>var.trace_variable("w", on_write)
> 
> 
> Tell the StringVar to call the function on_write() every time its value is
> changed. So every time the user edits the data in the Entry, in turn the
> Entry changes the data of the StringVar, which calls the on_write()
> function which may or may not change the StringVar -- and that change is
> reflected in what the Entry displays. This smells like an endless loop, but
> so far we seem to be lucky...
> 
> If you look again at Fredrik Lundh's ValidatingEntry, you will find all the
> elements explained above packed nicely into one class, with the extra
> refinement that he keeps another copy of the value which is used to restore
> the old state when the new value is found to be invalid.
> 
> Peter
> 

Thank you, man! You should write an tutorial to Tkinter or something 
like that.