trace tkinter
from Tkinter import *
#------------------------------------
def my_tracer(a, b, c): # trace send 3 arguments to my_tracer
#print a, b, c
# using StringVar to get and set text
new_text = e1_var.get() + e2_var.get()
t_var.set(new_text)
#------------------------------------
top = Tk()
#---
t_var = StringVar() # or StringVar(top)
t = Label(top, textvariable=t_var)
t.pack()
#---
e1_var = StringVar() # or StringVar(top)
e1_var.trace('w', my_tracer) # run my_tracer if value was changed (w = write)
e1 = Entry(top, textvariable=e1_var)
e1.pack()
#---
e2_var = StringVar() # or StringVar(top)
e2_var.trace('w', my_tracer) # run my_tracer if value was changed (w = write)
e2 = Entry(top, textvariable=e2_var)
e2.pack()
#---
top.mainloop()
#-----------------------------------