python 2.7 - Tkinter callback in a class

Question

I understand concept callback button in function and procedure.How to apply this code in class?I have problem about callback button in Tkinter for get value from class.I can't access callback method.Eror program = callback not define.

from Tkinter import *
class Program: 
    def __init__(self):
         self.root = Tk()
         self.root.title("Sample")
         self.display = Entry(self.root)
         self.display.grid(row=1, column=0, columnspan=5)
         Button(self.root, text="Proces", width=5, foreground="blue", command=callback).grid(row=4, column=0)


    def callback(self):
         print hello

program = Program()

mainloop()

Answer 1

What you need to do is use: command=self.callback instead of command=callback.

Also, it should be print "hello"