Here's an example I just made:

#-----begin------

from Tkinter import * #part of standard library

def reverse():

    s=entrybox.get() #get text from entry widget

    entrybox.delete(0,END) #delete text from entry widget

    entrybox.insert(0,s[::-1]) #reverse text, insert it

entrybox=Entry(root) #create an entry widget

entrybox.pack() #display it in the window

button1=Button(root, text='Press Me', command=reverse)

button1.pack()

entrybox.insert(0,'type here') #add a default value

root.mainloop()