from Tkinter import *
from math import *
root = Tk()
top = Frame(root); top.pack()
Label(top, text='Define f(x):').pack(side='left')
f_entry = Entry(top, width=12)
f_entry.pack(side='left')
f_entry.insert('end', 'x')
Label(top, text=' x =').pack(side='left')
x_entry = Entry(top, width=6)
x_entry.pack(side='left')
x_entry.insert('end', '0')
s_label = Label(top, width=9)
def calc(event=None):
f_txt = f_entry.get()
x = float(x_entry.get())
res = eval(f_txt)
global s_label
s_label.configure(text='%g' % res) # display f(x) value
x_entry.bind('', calc)
Button(top, text=' f = ', relief='flat',command=calc).pack(side='left')
s_label.pack(side='left')
def quit(event=None): root.quit()
root.bind('', quit)
root.mainloop()