[Tkinter] messed callbacks

[Giacomo Boffi]

i have this test program (that i already posted on it.comp.lang.python)

----[ test.py ]
from Tkinter import *

def output(s):
  print s

def doit(fr,lst):
  for c1,c2 in zip(lst[::2], lst[1::2]):
    subframe=Frame(fr)
    Label(subframe,text=c1+' <-> '+c2).pack(side='left',expand=1,fill='both')
    Button(subframe,text='>',command=lambda: output(c1+'->'+c2)).pack()
    Button(subframe,text='<',command=lambda: output(c2+'->'+c1)).pack()
    subframe.pack(fill='x',expand=1)

root=Tk()
fr=Frame(root,relief='raised',borderwidth=5).pack(fill='both',expand=1)
doit(fr,['pippo','pluto','paperino','zio_paperone'])
Button(root,text='Q',command=root.destroy).pack(expand=1,fill='x')
root.mainloop()
----

when i execute it from the command line, click on the 4 buttons from
top to bottom and quit (you can do the same), this is what i get

aiuole: python test.py 
gray->black
black->gray
gray->black
black->gray
aiuole: 

as you see, the button 0 and 1 have callbacks different from my
expectations, as my script was intended to give

cyan->blue
blue->cyan
gray->black
black->gray

why the messed callbacks? what's the right thing to do?

tia, 
                                                                g
-- 
l'amore e' un sentimento a senso unico. a volte una via comincia dove
finisce un'altra e viceversa                       -- Caldana, in IFQ