[python] limiter un entry

#!/usr/bin/python
# -*- coding: iso-8859-1 -*-

import Tkinter
import Pmw

class simpleapp_tk(Tkinter.Tk):
    def __init__(self,parent):
        Tkinter.Tk.__init__(self,parent)
        self.parent = parent
        self.initialize()

    def initialize(self):
        self.myentry = Pmw.EntryField(self.parent)
        self.myentry.grid(column=0,row=0)
        Tkinter.Button(self,text=u"Cliquez moi", command=self.OnButtonClick).grid(column=1,row=0)

    def OnButtonClick(self):
        print "La valeur est:", self.myentry.getvalue()

if __name__ == "__main__":
    app = simpleapp_tk(None)
    app.title('Mon programme')
    app.mainloop()