Python GUI - tkinter

#!/usr/bin/env python3

from tkinter import *

def show():

   global b1

   b1['text'] = "Hello World!"

root = Tk()

root.title('Hello')

frame = Frame(root, width=200, height=150)

frame.pack()

frame.pack_propagate(False)

b1 = Button(frame, text='Click', command=show)

b1.pack(expand=True)

root.mainloop()