#!/usr/bin/env python
from Tkinter import *
import math
root = Tk()
top = Frame(root)
top.pack(side='top')
hwtext = Label(top, text='Hello, World! The sine of')
hwtext.pack(side='left')
r = StringVar()
r.set('1.2')
r_entry = Entry(top, width=6, textvariable=r)
r_entry.pack(side='left')
s = StringVar()
def comp_s(event):
global s
s.set('%g' % math.sin(float(r.get()))) # construct string
r_entry.bind('', comp_s)
compute = Label(top, text=' equals ')
compute.pack(side='left')
s_label = Label(top, textvariable=s, width=18)
s_label.pack(side='left')
root.mainloop()