python - 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 to be able to edit it but stay in the 20 characters limit. In or order to keep the code light , should I use a regex , a loop or check the entry with an event ?

Here is my code:

import Tkinter
from Tkinter import *
import tkFileDialog

root = Tkinter.Tk()

edit1    =StringVar()
edit2    =StringVar()
s = StringVar()


s = "GATACACGCGCGCGTATATATTACGCGCGCGATACA"



lb01=Label(root,text="sequence1")
lb01v=Entry(root,textvariable=edit1,width=20)
lb01v.delete(0, END)
lb01v.insert(0, s[6:20])

lb01.grid(sticky=W,row=1,column=1)
lb01v.grid(row=1,column=2)


lb02=Label(root,text="sequence2")
lb02v=Entry(root,textvariable=edit2,width=20)
lb02v.delete(0, END)
lb02v.insert(0, s[0:6])

lb02.grid(sticky=W,row=2,column=1)
lb02v.grid(row=2,column=2)

root.mainloop()

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()