entry widget won't validate

[Mike]

I'm trying to arrange for an Entry widget to check whether its data
is all digits and whether the number represented is small enough.
The validate function seem to be called once at startup and not
afterwards:

import sys, Tkinter, tkFileDialog, tkMessageBox

tk=Tkinter
tkfd=tkFileDialog

...
class Functor :
    def __init__(self, func, data) :
        self.func=func
        self.data=data
    def __call__(self) : return self.func(self.data)

...
class Levels(tk.Frame) :
    def __init__(self, parent) :
        tk.Frame.__init__(self, parent)
        self.tv=[]
        for rgb in range(3) :
            self.tv.append(tk.StringVar())
            e=tk.Entry(self, textvariable=self.tv[rgb], width=5,
                       validate='all',
                       validatecommand=Functor(self.vc, self.tv
[rgb]) )
            self.tv[rgb].set(str(levels_av_max))
            e.grid(row=2, column=rgb)
            lab=tk.Label(self, text='RGB'[rgb])
            lab.grid(row=1, column=rgb)
        doit=tk.Button(self, text='set levels', command=self.pushed)
        doit.grid(row=0, column=0, columnspan=3)

    def vc(self, arg2=None) :
        print 'vc:', arg2.get()

The print statement run 3 times at startup.
Editing an Entry does not cause any printing.
Any ideas?