Limiting entry on a tk widget

Question

I have trouble finding a way to limit the entry length of entry widgets, I would like to limit it to 20 characters, i.e. when I click on a sequence or the other I would like

Answer 1

Ok I did try with the trace variable, on a short piece of test code , this is excactly what I was searching for !! I like the fact you can prototype so easily in Python ;)

def main():
    pass

if __name__ == '__main__':
    main()

from Tkinter import *

def callback(sv):
    c = sv.get()[0:9]
    print "c=" , c
    sv.set(c)

root = Tk()
sv = StringVar()
sv.trace("w", lambda name, index, mode, sv=sv: callback(sv))
e = Entry(root, textvariable=sv)
e.pack()
root.mainloop()