StringVar.py

try:
    #py 2.x
    from Tkinter import *
except ImportError:
    #py 3.x
    from tkinter import *


app = Tk()

text_var = StringVar()
text_var.set("testando")

frame = Frame(app, relief=RIDGE, borderwidth=2)
frame.pack(fill=BOTH,expand=1)

def trocar_texto():
    text_var.set("foi testado")


label = Label(app, textvariable=text_var)
label.pack()

button = Button(app, text='click', command=trocar_texto)
button.pack()

sair = Button(app, text='Sair', command =app.destroy)
sair.pack(side=BOTTOM)

app.mainloop()