trace in tkinter code example

Example: 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()

#-----------------------------------